Why is Dawson fighting Johnson again?

[Cleon]

Johnson is the only fighter HBO would approve. So it Dawson want to make the big bucks, he has to do what they say.

True
There was also a calling for a return from boxing fans & media alike
Before Chad could move on he had to deal with the unfinished business

Which makes me wonder why the hell he had to fight Tarver again :P

Tarver invoked the rematch clause he already go his ass kicked but wanted a second helping I guess
I think Dawson & his team were trying to avoid Johnson but couldn't get a bigger fight