Not hard to imagine why. I'm guessing the racial stuff and mistreatment were bad for that guy, but not insanely bad enough on a long-term, day to day basis, where it would be worthy of a lawsuit. I'm sure, considering how Tom came to his defense, that he probably is fond of Tom, and considering they had a long, intimate working relationship and probably a close friendship, he might have preferred to handle his pursuit of back pay in a way that harmed Mr Benson as little as possible. When that didn't work out, then it was time to stop pulling punches and bring out the big guns.
Also, $50,000 a year to work 10-14 hour days as a millionaire's personal assistant seems like chump change.