The same reason the media will ask Republican candidates about evolution or the age of the earth (and the same reason they won't ask Democrats about partial-birth abortion). It's a strategy to make these candidates look "extreme" to swing-voters who don't pay much attention to politics.
The press has their marching orders and they will spend the next 18 months asking stupid “gotcha” questions, which will then be replayed over and over again on the network news...
If you want to know who the Democrats are really worried about, just take a look at which candidates the MSM are trying to brand as nut case extremists (Walker, Cruz, Paul). Then take a look at which ones are being called “statesmen” and “serious contenders” (Bush, Christie, Lindsey Graham). Those are the ones they know they can beat so those are the ones the media has been told to leave alone until after the primaries.
It’s not exactly rocket science - it happened with Bob Dole, John McCain and Mitt Romney...but the RNC still doesn’t get it and Karl Rove just gets richer and fatter making promises that never seem to come true!