When someone mentions Mixed Martial Arts, the first thing that comes to mind for most people is UFC because it is the face of the sport.
MMA is one of the fastest growing sports in the world and this is largely because of how much UFC advertises itself.
There are commercials about fights, there are ads in magazines, ads on the radio, billboards displaying the newest UFC fights. If there were only small organizations like Strikeforce the sport wouldn't be as popular.
MMA owes its popularity to UFC for making it known but not everything about it is positive. Many parents of interested trainers don't approve of it and there are several reasons why.
But without UFC, MMA would still be run by minor groups trying to make it big and there wouldn't be as many members. Who is to say that another minor company wouldn't be in UFC's position right now, but it's hard to tell.