Because snooker is a better cue sport, and is more enjoyable for the spectator. I guarantee that 90% or more of people that say they'd rather watch pool have probably never in their lives watched an entire snooker match.
Part of the reason it's more enjoyable is that unlike pool, the games don't all blend together. Also if you look at the ratio of offense to defense and compare it to US pool games, it's easy to see that snooker has the most even distribution. The only game that's close is one pocket. But it's still far less enjoyable for spectators.
8/9/10 ball - 90 offense / 10 defense
14.1 - 85 offense / 15 defense
One Pocket - 30 offense / 70 defense
Snooker - 60 offense / 40 defense