I think it depends on the game.
Sometimes, you get PC releases a little bit later because some developers (I recall reading somewhere that Capcom is one) don't start on the PC version of their game de jour until the console version is finished. This would create delays, obviously. Primary reason for this one would be, yeah, console gaming has (I think) the larger audience, so they're going to pander to that crowd first.
With some games, the port is later because of something being added to it rather last minute. Arkham Asylum, for example, was later because the devs added PhysX to the PC version, which bumped the shipping time a little bit.
Other times, (and I'm just speculating on this one), you have to make sure the game runs properly at the minimum spec, the recommended spec, and anything in between. There are a lot of potential hardware variations to PC gaming rigs, and you have to make sure that nowhere along the line does your game conflict with these. (Like I said, I may be talking out of my ass as far as that one goes; just a speculation). You couple that with starting on the PC version later, or choosing to add something that you weren't going to initially, and, yeah, you'd get a later release date.
But, like everything else in life, what it comes down to is money. [A lot of] publishers and developers see the console market as being the better money-making market, so that's going to be their focus. It sucks if you're primarily a PC gamer, having to wait that indeterminate amount of extra time for the game to be released on your platform of choice, but I don't think much is going to change anytime soon.