…to mock the Romneys for having a black grandchild?
The main reason: their audience is so tiny and leftophile that the panel members forgot they were on TV and thought they were talking amongst themselves. Although they’ve now issued apologies and semi-apologies, it probably wasn’t until liberal CNN criticized them that they realized they might have done anything that could be considered even the least bit objectionable.
After all, the Romneys are infinitely mockable to the left, and just as with Sarah Palin the mockery goes on even though the campaign has ended. Unlike Palin, Romney has retreated from public life, but that doesn’t stop the left from continuing with the fun game of making fun of everything he and his family do.
Plus, the child is obviously going to be a black Republican. Everybody knows that black Republicans are not really black people and are therefore fair game.
[ADDENDUM: And in an only tangentially-related article, Joan Walsh bends over backwards to prove she's not one of those bad white people. She's a good white person. So please don't hate her.]