It's not hard to figure out why they couldnt use them.

Because there was none of ANY of these to allow for it....

Mass
Power
Volume
Money
Time

A 'balloon or two' would require...

Computing power independant of the rover
Independant power source,
Communications systems
The balloon or balloons themselves
The gas to inflate them.

That landers had ONE job...to put the rovers on the ground safely. They had to be engineered to do just that.

Take a look at the images of the empty landers...there isn't the space for ANY of the systems that would be required to allow that sort of addition, let alone the balloon itself.

Balloons on Mars are a difficult one ( given the very very thin atmosphere ) but might have science value...HOWEVER...there was never any way that such a thing could be added to the lander structures for the rovers.

Doug