Why do people drop out? In my opinion, because it's not fun any more. It comes down to the issues we were talking about earlier this week in the "teaching real flying" thread.
"Aimless" local flying post-PPL gets dull after a bit, and as BlokeInAnArcher said, VFR flying in the UK isn't really much practical use. If you have to get somewhere (rather than if you just want to get somewhere), and if you have to get back home again, you will drive / take the train / use the airlines.
There is plenty of fun flying to be had, at fairly reasonable prices, but it isn't offered by the flying clubs.
Some people stop because they can't afford it, but in most cases if you can afford to get the licence, you can afford to keep it afterwards.