Bethesda has a long history of making most of their DLC a timed exclusive to the X-box 360, usually leaving the PS3 and the PC out in the cold for months at a time until they get around to releasing it for everyone or throwing it onto a Game of the Year disc so that you'll have to repurchase the game if you want to get ahold of it. What're the odds that this same cycle will repeat itself? How will you react if everyone can play the next chapters of Skyrim months or even a full year before you can?
It's hardly a long history. Morrowind did not come out for Xbox until a bit after it came out on PC because they needed time to port it. It was their first TES game that went to any console and the only DLCs were either free or came in the way of large expansions on the retail market. Oblivion was the first time we had DLCs that were charged for and there was one full blown expansion and one DLC of considerable size. The DLCs for it were time exclusive for PC and Xbox 360.
As long as there is a substantial amount of money offered for exclusive rights timed or not, to make it worth the loss of up front sales of the DLCs I imagine they will take the money and continue to have timed exclusives.
PC also got those timed exclusives for FO:3 and ony with FO:NV did pc not get them first time around. I would imagine (though I have no way of knowing for sure) that pc not getting those had something to do with switching distribution from GFWL to Steam.