Why do new PPLs give up?
1) They never intended to carry on; getting the PPL was a challenge, and they've done it.
2) They've spent all their dosh getting the PPL; the school convinced them they'd do it in around 45 hours, and it took them 60-70 (national average) and therefore loads more money.
3) The school/club won't let them hire an aircraft for longer than an hour at a time, and the novelty of boring holes in the sky in the local area at £100+ a time wears off quite soon.
4) The family doesn't share their enthusiasm for flying, and would like them to join in family activities on the occasional weekend - the only sunny one of course.
5) They don't like flying alone, and no-one else wants to come.
6) They'd hoped to costshare with family and friends. But for all the prospective passengers it was a one-off, and they don't want to go again.
7) The school/club isn't a particularly good place for social activity, and they can't find other pilots to fly with.
8) They can't afford a share in an aircraft, and anyway, there aren't any going at the moment, espeically as insurance rules probably preclude under-100-hour pilots.
9) They can't afford to buy an aircraft, and wouldn't know where to start looking.
10) As a result of all/some of the above, the new PPL doesn't get enough practice, and doesn't feel safe when he/she actually does get to fly. Two years go by - it's revalidation time. The no-longer-new PPL hasn't flown 12 hours, dreads flying alone, has no-one to fly with, and doesn't want to do another Skills Test for an activity that hasn't turned out to be as expected. So of course he gives up!
I'm sure there are other reasons, but the above makes sense to me. It might have happened to me - but I found another PPL who wanted to fly with me and visit new airfields, the school let us take away an aircraft for a whole day, and I don't have a family to complain about all my flying. And then I discovered helicopters...but that's another story.