Well, two basic reasons:-

1) The original search came up so blank there wasn't even a sheet of paper to write "blank" on.

2) Thinking about the results suggested that we had got ourselves into a sort of scientific "20 questions" game in searching for life on Mars:-

In 20 questions, you are allowed just that - 20 questions to find a hidden piece of information, and each question can be answered as "yes" or "no". The puzzler chooses an object and the questioners try to guess it. Another version is called "animal, vegetable, or mineral?" and the puzzler starts by declaring which of the three it is. (Mars wasn't so kind to us, so we started from scratch)

Strategies in this game require you to start with general questions that are efficient in sorting through all the possibility space, and focus in right at the end on specifics. For instance, if the puzzler chooses Napoleon Bonaparte, the questioners might start
"was this thing alive once alive or still living?" and get a yes.
"Was this thing warm blooded?" - yes.
"Is this thing still alive?" - no.
"has this thing got legs?" - yes

Eventually you go through mamalian, biped, human, male, military leader, famous historical figure, frenchman, and if you've been efficient and have a question or two left, you can THEN ask "are you Napoleon Bonaparte?"

A major strategic failing in this game is to try to outthink the puzzler. If you know that he has a penchant for famous military leaders with a dictatorial bent, you might try a suite of likely targets such as:-
"are you Adolf Hitler?"
"Napoleon Bonaparte?"
"Josef Stalin?"
"Saddam Hussein?"
"Ghengis Khan?"

(Regrettably, history is full of charismatic dictatorial military leaders)

You very quickly use up your precious supply of questions and by the time you back off and start a more general approach, it's too late.

What if the chosen object was "Adolph Hitler's moustache" or a famous painting of Oliver Cromwell? The puzzler is still on his familiar territory, but the questioners are doomed because they haven't imagined that it might be a part of a person, or a picture of one, rather than the whole person.

Anyway, we're in a similar position with the search for life on Mars. Viking was the "Are you Adolf Hitler?" question*, and for backup we asked the same question twice. We got the same answer which was something like "Adolph Who?".

It took a decade or so to really understand the implications of Viking - that the surface is organic poor, oxidising, irradiated, and an unsuitable place for terrestrial-style life. Some people still disagree about this interpretation of the answer.

What we are doing right now is to ask the general questions about Mars. For a start, we're asking what sorts of fluids were there on Mars, and where and when. We've found subsurface permafrost on modern Mars in the temperate and polar regions, signs of glaciers on the surface in it's past on the Tharsis volcanoes, evidence of intensely acidic sulphate fluids (aka battery acid) in the distant past at Meridiani and perhaps elsewhere, and a whole bunch of answers we still don't understand. We have to integrate this knowledge, design new questions (missions), and learn as we go along.

- and remember we have had a grand total of only 3 landers since Viking:- Sojourner and (Oppy & Spirit) (which are clones so don't really count as two seperate questions). We've also had orbiters, too.

In my analysis, I think it is hugely premature to search for active life on Mars. We don't know whether it is buried in the polar ice, buried below volcanic rock, buried below sedimentary rock, or buried somewhere we haven't even though of yet. We're fairly sure that IF it exists, it is buried. We don't know how deep - it could be 1 metre or 1 mile, or 10 miles or more. We don't know how to drill even 1 metre on Mars. We don't know where to drill, what tools to drill with, what measurements to take while we drill, and what NOT to do that could wreck our chances of finding evidence of life, even if it is or was there.

Sure we could send an amino acid detection biochip, but it would likely come up as empty as Viking, but with equally odd aspects of its measurement that showed we didn't even understand what we are measuring.

In short, it's a damn difficult question/search. NASA (i.e. the US taxpayer) has put billions of dollars into it over 40 years. The USSR dumped a load of roubles in spectacularly unsuccesful Mars missions. ESA is beginning to spend big money on Mars. Colin Pillinger asked the "Saddam Hussein" question with Beagle 2, but was in such a hurry that he blurted out his question without getting the syntax right, and so the answer was "pardon?", but that still used up a question (mission)

The really important thing, having flubbed with Viking, is not to rush and ask the "Napoleon Bonaparte" question. We need to find out which questions we DO need to ask, and right now we don't know.

So that's why we're not actively searching for life on Mars, right now.

But we will, probably, soon, if we don't run out of questions (missions).

*At the same time as Viking was being designed at a cost of over US$1 billion to ask the "Adolph Hitler" question, a scientist called James Lovelock went and asked "are you an extant cellular life form?". For US$40,000 he measured the atmosphere of Mars and showed that it it was essentially in chemical equilibrium. Earth's atmosphere is wildly out of chemical equilibrium - caused by the metabolism of all the cellular life on Earth. The absence of such a disequilibrium implied that there are no great metabolic fluxes on Mars.

Lovelock was dismissed in his day (partly because he wasn't employing enough other scientists on his limited budget), but his ideas are now the basis for the search for life beyond the Earth, with plans to build detectors that can spectroscopically analyse the atmosphere of planets orbiting distant suns (once we build things powerful enough to *see* those planets against the glare of the star). If we find disequilibrium, it suggests a possibility of life, depending on how strong the signal is. (Some false signals can arise naturally - e.g. trace ozone in the CO2 atmosphere of a dead planet, from photolysis).

Following this line of reasoning, we are all intrigued by Mike Mishna's results of disequilibrium atmospheric methane detection on Mars which seems to be a muffled "yes" to the question "MIGHT you be a very small or deeply buried biota?".

The jury is still out on methane on Mars, but we certainly need to ask a better Methane question to find where, when, and how much methane there is. Thios may require a future mission to carry a specially-tuned spectroscope to optimally detect methane, and other plauisibkle gases that may accompany it.

Another thing we need to do, for example, is to send a mass spectrometer to meridiani and to measure the Sulphur isotope distribution of the sulphate rocks there. On earth, bacteria flourish in acidic hot springs, which is approximately what the Meridiani deposit represents (we argue about whether or not there was an open body of water, but there sure as heck was wet stuff somewhere). Bacteria cause huge changes in the S isotope distribution so we should compare S isotopes from Martian Meteorites with the S isotopes at Meridiani. - another question (mission) and quite a specific one - "Were there sulphate-metabolising organisms at Meridiani when the sulphate deposit was formed?"