#BasedMayne was a bigger box office draw and a more reliable actor than Robert Downey Jr - remember, this is when RDJ was JUST getting back in the saddle after his period of drug abuse and rehab.

So yeah, Terrence Howard was the highest-paid performer in that film.

When it came time to do Iron Man 2, Marvel intended to pay RDJ a LOT more money than on part 1, as he deserved it, and were looking to find ways to renegotiate Terrence Howard to a lower salary with less screen time (Howard had signed a three-picture deal - at a certain per-picture salary - with Marvel to appear in all the Iron Man films).

Howard and his people balked, and add to this that apparently director Jon Favreau didn't exactly enjoy working with #BasedMayne and he was eventually replaced.