Don't you think that slot games are difficult to win ? I know they are entertaining but I really find it hard to win them. One of the games which i like most is the crash games and Roulette. Card games are also very popular and entertaining like Baccarat.
Harder to win because the house edge is higher than other games such as crash and even roulette.
Slots are only for those who is willing to take higher risk for higher return, slots give you chance to win many possible results in a single spin which is not available in some other games.
I would not say that only one game is more popular than other as every gambler tend to like different games and those who are regular gamblers will play almost all the games.
By the way, poker games are also very popular and those who get addicted to poker, they won't play other games

But we cant deny the fact that most gamblers has their own favorite game to play, they may have few favorite games but I'm sure they have one as the most favorite.
This fact makes a fact that there is one game which become more popular than other games, can be based on the number of players or can be based on the average wager on the game worldwide.
Your last sentence already contradict the previous sentence though