As far as we know, there is only one way of transmitting information over interstellar distances, via electromagnetic radiation. There may be other ways, but until we know what they are, we haven't got a clue where to start.

There are only two types of electromagnetic radiation which could give away the existence of ETI:

1) Radiation released as a by-product of their industrial or other activities, such as leakage from radar systems, etc.
or
2) Radiation deliberately generated in an attempt to get someone else's attention.

The former will probably be very difficult for us to recognize as being an artifact, we may mistake it for some natural process or it may not carry very far because it isn't optimized for communications purposes. The latter we have plenty of reason to believe we will be able to recognize as a deliberate hailing beacon. To wit:

a) It will be of a type which will be obviously not the result of a natural process or likely to be mistaken for one.
and
b) It will be such that it will propagate easily through the interstellar medium and support a high signal to noise ratio over long distances.

No matter what ETI is like culturally, or its motives, or how it chooses to modulate its signal or broadcast information, signals of type 2)b) are what we should be looking for for purely physical reasons which are totally independent of their level of technology or psychological makeup. Right now we know of only one form of radiation which fits these criteria: narrow band microwave radiation in the frequency range known popularly as "the waterhole".

There may be other ways or means, but until we figure out what they are, there's not much point in worrying about it. Even if they do have other technical means of transmitting these signals, the microwave water hole is still probably a good choice because it is one which any species with radio telescopes and astronomical interests is likely to be monitoring anyway, for other reasons.

If after only 50 years of intermittent listening we haven't heard anything is quite easy to explain: There are 400 billion stars in our galaxy alone, we haven't been listening anywhere near long enough to have any chance of intercepting a signal. Fifty years is nothing.