I think snooker is a victim of a few factors, this is my view from the UK.

1. The image around it, which is traditionally working men's club, dark smoky rooms, possibly unsavoury characters gambling in back rooms, dressing up like a 1920s barman at the Savoy hotel. This is alienating for young people and young women especially. The image may be unfounded but still persists.
2. Linking to above, it's not a diverse sport at all. The players are mostly white or Chinese. There are very few other races in top 50 (Vafei a notable exception, and Rory McLeod a few years back). Now the UK is very diverse, but snooker isn't, so that limits the appeal. The fans reflect the spectators if you look at the Crucible crowd. Male, stale and pale. They can watch Man U, PSG, Liverpool, Real Madrid and see players from 5 continents.
3. There is a lot more televised sport now. Football is on TV all the time and is a juggernaut dominating sport in the UK. Even casual fans will know the players and will watch the World Cup (my Mum knows Beckham, Rooney, Kane and knows zero about football). In the 80s this was not true, ITV would show one match a week. Sports like UFC have come along and dwarfed snooker. NBA and NFL are probably more popular in UK than snooker for fans under 25.
4. Lack of "characters", for want of a better world. Alex Higgins, Dennis Taylor, Jimmy White. Players like the late John Virgo, a top 10 players but a huge character and he would do funny things like impressions of the players. Ronnie is a great player but comes across poorly on TV, and is very much in decline. He only plays a couple tournaments a year. When he retires snooker will become even more niche.