Well they're talking about it for reasons I'll list
It's the VP's child, in America we like to see our President and Vice President as a reflection of ourselves someone we could have a beer with somebody like us. This doesn't mean they're good at running the country though... So if a majority of people believe in 'family values' like sex before marriage, religion (there will never be an atheist president) etc. We frown upon extra marital affairs, etc. Voters pick people they like, it's never about ideas...
McCain picked a person he had only met once before in his life, she comes from a state most Americans only go to once in their life on a cruise, if at all. Nobody knows anything about her, so these revelations are huge in defining who a person is in the public square. You as a politician want to define yourself and not have anything define you first because once public opinion shifts to a negative view point, you're done.
So in that respect it's a major news story.