I think it's mostly the cost of playing the game. Same around me, all the halls are closed, there'd be huge setup costs involved, plus insurance, light, maintenance, staff. They aren't cheap facilities to setup and keep open.
The cost of playing darts is much much lower, for less than £100, you'd have a very good home setup. Costs next to nothing to go play in the local pubs and clubs either. You can get good darts relatively cheap and just upgrade and add to them over time.
Snooker, you're looking at the cost of the cue, rental fees for tables amongst other things. It's just too expensive for most people to even bother with. If you wanted to setup at home it requires a lot more space and hell of a lot more money.
Dumbing down the game to make it attractive to a younger audience may increase viewership, but if there's nowhere to play or the cost is too high then it won't really matter when it comes to increasing participation.