BJJ works in MMA because headbutting isn't allowed.
Imagine if the top guy was allowed to grab the bottom guys arms and headbutt from the top position.
It would be all over

In the beginning was allowed and Royce won 3 times.
Not all BJJ movements are allowed in MMA.
Heel hooks...toe holds..spine locks.
Controlling small,weak portions of the body (fingers ,wrists,ankles) wich leads to controlling the connecting strong points (arms,legs).
Another little thing if you grab onto an earlobe,you will find that is has a nice grip for you.and grabbing a person's ear is much more effective than a fishhook (you can be bitten and the lip is much more slippery).
Try pinching your knees together as you pop up your hips while someone is in your guard? If you hit it right on the bottom rib it hurts a lot.
Under hook behind his back you can do the same thing, but it's a lot tighter.
How easy is to break a neck?
DJJ is real. Dirty Jiu-Jitsu