Why is darkrp so popular?

In all honesty, the game is very customizable and quite elaborate. The source engine does have its limits, but it allows you to tinker with things other games don't. With Gmod, you still have that cstrike:source feeling when you're running around so I think that's why people are drawn to DarkRP mostly.

Not to mention it's a very social environment you can creatively express yourself in.