Pool and snooker are two very similar games. They both involve using pool cues to pocket different colored billiard balls on the table. Both require different billiard supplies, are two player games that involve the calling of foul shots and require skill in aim and focus. Yet, they are entirely different when it comes down to the details.
Snooker is a thoughtful, slow moving game that requires patience. One frame can go on for an average of one hour. The table is much bigger than the pool table - two meters wide and one meter in length. Because of the size of the table, break building (i.e. an unbroken series of pocketing the shots) is very difficult. It requires a great amount of planning. Snooker players need to be experts at using the spin on the billiard balls so that they can play the shot in such a manner that they end up positioning the cue ball for the next shot. The billiard balls are usually hit smoothly and not whacked across the table. Scoring is on the basis of the balls, each of which has a different value.
Pool tables, on the other hand, are smaller - a meter in length and less than a meter wide. The pockets are larger and the pool table felt used is much coarser (and hence requires more force while playing). Here the rules are less strict and the pace is quicker. It also doesn't need devices like the spider, which allows for shots that are too far to be reached by an average human. There is no scoring as such; the person who finishes and pots the black ball first wins.
In a friendly non-competitive environment, it's easy to see why pool has become more popular than snooker. For one, snooker takes much longer to play than pool. Two good pool players can clean up a pool table in 5-10 minutes. That's not the case with snooker.
Snooker is also more difficult and hence never caught up in terms of popularity with the general players. In terms of excitement, pool also gets an upper edge here. Because of the duration of the game, players tend to enjoy playing it much more. Pool also makes for a good spectator sport due to this reason.
Pool parlors and bars also have a big role to play in this. At various times, pool parlors have sprung up in various places all across the world. They proved to be profitable businesses for entrepreneurs and people frequented these places as it had a fun sport involved in the social process. Remember: pool was originally a betting game, and got its name because players and gamblers 'pooled' their money on the table.
Ultimately, both games have their pros and cons in their own respective fields. Yet, it's clear to see why pool has gained popularity.