Most things have been mentioned, I guess. I think one big thing is that there is no "instant gratification" in pool, it takes too much time {and now, money} to learn the game to where you can even play at a decent amateur level. When we were back in NYS to visit in March there were no young players in either of the rooms we visited and I understand they are few and far between. One place we visited had 16 guys in it at the time, two had jobs and the rest were retired or, like me, on Disability. One place has twenty-some tables and I don't know how they can afford to stay open.
It's a shame that the biggest force in amateur and entry level pool is the APA. They've done a lot to hurt pool long term. Contrary to what is claimed which is that they've done a lot because they are the largest league ever.
My reasoning for this is simple...without beginner level leagues, newbies to the game have no competitive or fun way to participate without getting crushed for a long time. That's just how it is. Pool has a learning curve that takes years. Makes sense, but there's more to it.
Had the BCAPL or whatever other league other than the APA been dominant, pool would be in a better place. The APA may be the biggest, but they also taint pool with the unfavorable culture of pool the APA promotes. That sleazy, bar style pool that is almost as focused on getting drunk as it is on pool. The idiotic rules. The overly-exploiting format and system. It's just bad all around.
Thousands upon thousands have joined the APA. But do they stay? Aha! The APA's numbers are down, and for a good reason. Over the years I've seen so many players do the APA thing, get sick of it, and then leave in disgust. However, people who started in BCA or a similar type league system tend to stick with it for a long time. There are many reasons for this, which are beyond the scope of this thread.
The BCA has a better "culture" of pool within it. It's more sportsmanlike, it's more civilized, it's just higher class all around. It's a better example of pool for newbies. People can still drink and have a good time. Also, it doesn't punish higher skilled players driving them out. There's better integration between beginners and skilled players, meaning there's more interaction and players develop better and faster. You need mentors and you need to be around good pool to get good. It doesn't exploit and downright cheat so many players and teams by the inherent structure of the league.
The APA isn't the biggest because they're the best for pool. They're the biggest because they're the best at selling what they've got and spreading like a cancer across the land. The best at filling their own pockets with their players err, I mean customers money. Their franchise system is effective, and LO's have many times used bullying tactics to harm competing leagues and try and monopolize an area.
They have huge turn around with so many people joining, but also quitting, and quitting for good. The corruption, the BS, the sleaze all turns off a lot of people from pool. Why aren't those people ever considered? Instead, there is nothing by endless praise for the APA with their "300,000" members. These players aren't skilled enough to enjoy higher level pool and open tournaments, but don't want to stay with the APA. What do they do? They go golfing or something.
Anyway, by being one of the biggest entry points of pool for many beginners, the APA sets tens of thousands of players in the wrong direction, turning them away from perhaps a lifetime of playing pool, toward being players who get sick of the novelty because that league sucks. First impressions make a big difference. The APA is just plain ugly.
That's how it hurts pool long term.