Because Cosby went to the Twitter World and invited them to make Meme's of him. After a few used it an opportunity to criticize him in this way, it took off with many others. Too much exposure and the media can no longer ignore it.
Bill Cosby Memes Backfire After Internet Focuses on Rape Allegations
http://www.eonline.com/news/596722/billy-cosby-memes-backfire-after-internet-focuses-on-rape-allegations
That's all true about the memes and the Internet.
But given the way the media works they probably could have ignored it.
I suspect they viewed Cosby as another story they could cover to avoid having to say anything critical about he administration.
That doesn't mean Cosby isn't a bad guy, though.