Not counting the guys with self multiliation/mutiliate other moves, I'd say most people in POT at least seems to enjoy tennis. You can even argue it might be fun to abuse your opponents. So no I don't think 'have fun' is really that hard to do once you know the trick.
The first two doors are achieved by talent + hard work. For example Chitose says while exploring Muga he hooks himself up to all kinds of machine and take himself to the limit of his physical ability to try to figure out what lies behind. In light of this, it's possible that everyone assumes that since first two door = hard work + talent, the last door must require even more hard work and talent, but of course the more you work toward TnK the more you're away from it because the key is to enjoy tennis.
So in original POT, before Nanjirou come out and explain the secret I can see why nobody was able to do it. If Tezuka, Sanada, and Yukimura are supposed to be representative of the top tennis players in POT that you'd model your career after, then certainly you'd never come close to TnK while trying to play tennis like them, as there is absolutely nothing fun about the way they play tennis.
But since Nanjirou revealed the big secret, I don't see how the guys who already do seem to enjoy tennis (Akutagawa is an obvious example, but plenty other exists) won't just go 'whoa that's all I have to do to be invinicible? Awesome!' and then you suddenly have TnKs all over the place.
TnK is probably a main character special bailout, so that's another reason why we won't just see any random guy have it. But there's absolutely nothing hard about 'have fun' if you know that's the secret. I'm assuming, of course, that after Nanjirou revealed the secret the word gets around so it's just something only the guys present at the match would know.

