In general, the dragon theme is probably the most used theme in the world. I can't think of any theme that appears in more games in Las Vegas unless you think '7s' is a theme. In all dragon games, 5 dragons seem to be the most popular.
The ace333 is the company behind the Three Kings slot machine, which is known for creating slot games with Chinese themes. When it comes to Chinese culture, they have a lot of beliefs about luck - this may be why ace333 jumped out of this reason and offered many slot games different from the Chinese.