Cessna 172 is just the most common trainer, don’t know if it’s because of higher supply, easy access to parts and mechanics, or longer production years, I haven’t done any research but this would probably come into play.
Piper is a close second, I see them at a number of flight schools, usually newer aircraft, or a Piper Arrow to help get those complex hours for a commercial rating (not a requirement anymore).
Some other considerations are:
Low wing vs high wing
Single door
Requirement to keep an eye on fuel management
Piper has a shorter wingspan (easier to store in a hangar)
Many Pipers have no co-pilot brakes (can be added)
Piper generally feels like you are sitting down like a cool 16 year old in a race car, Cessna is more like your Mom driving a minivan.
Piper is cheaper to buy, I found the maintenance to be fine, and Piper uses less fuel than a Cessna. I can lean a Piper Arrow for 7gph or less (or more) whereas the club Cessna will be using 10-11gph.