Usually because they didn't get the point to begin with. They just thought that slapping a different person in there was ALL that it would take.
To be fair, sometimes they do get it right, though. Take BSG2. There was a huge uproar about turning Starbuck into a woman and, I have to admit, I was one of the ones being miffed about it, but mainly because I thought it was yet another "quota" thing.
Turns out it wasn't and I am now firmly of the opinion that BSG2's version of Starbuck is vastly superior to the original. Same goes for Boomer. Because the writers weren't just doing the idiotic "token" thing, they actually did their JOBS. And did them well.