IO540,
What does it? If you hang out in a heli school, you will find that everyone is super-enthusiastic...about helicopter flying. it's not because the machines are new or because it's cool; it's because it's real hands-on flying, with great views, you can land anywhere, and you're less likely to be scared of getting caught out by the weather because you can land if necessary. Most people aren't super-rich; they're often struggling to afford to fly, just like in f/w schools. They're just hooked on helicopter flying...and the novelty doesn't wear off.
So, thinking as I write, why do most f/w PPLs give up? Maybe it's because, after learning, f/w flying just isn't that exciting. You take off, trim it out, look at the view, then have all the skill and excitement at the end when you come to land. OK, you can go in for aerobatics. You can learn to fly tailwheel or other more interesting aircraft. You can get an IMC, you can get into long distance touring. But if you've got a plain vanilla PPL, perhaps the reason you give up is that it's actually quite boring.
Helicopter pilots don't find that. I don't think I've ever heard anyone say that they got bored with flying helicopters. And I don't know the figures, but I suspect that less PPL(H)s give up.