It really comes down to the fact that pool is more accessable than snooker, and I'm not referring to the ability to find places to play (although that is true depending on where you live).
Two people playing snooker for the first time will go perhaps 20 minutes without potting a ball that's not sitting over a pocket. Take the same two people and they will run 3-4 balls their first time playing pool.
I remember I hated snooker for a while, but played it now and then because I had the option at my university to play it $5/hr or pay $1.25 a game on bar tables. But all I wanted to do was run as many balls as I could, which didn't happen in snooker for me as I had a high break of around 20 at the time. But I later found Mordecai Richler "On Snooker" in the school library and his passion for the game was contagious.
As to which is harder, it really depends on what pool game you compare it to. Personally I think century in 14.1 is more difficult than a century in snooker. But I would say snooker is more challenging than 9 ball or 8 ball, it at least has more depth than those games.