He's looking for another payday because he knows MMA money is like working the hamburger line at McDonald's compared to being the CEO of Mayweather Promotions.
What will McGregor get for the Mayweather fight? I believe in the post-fight press conference, it was stated that the Mayweather fight's declared purse was 10x that of his best UFC purse. Would that be $30 million vs. $3 million?
Personally, I am disgusted with UFC purses - except for GSP. The rest suck. Fighting doesn't pay as poorly as bodybuilding, but both are way too low.

That being said, both are both market-driven.