With concerns of a potential market monopoly, Kevin Martin, chairman of the FCC, has stated that it is unlikely Sirius and XM would be allowed to merge. As they are pretty much the only option as far as satellite radio goes, with a single driving force behind them it becomes all to easy to price gouge customers who want it. Given that both companies have been losing money, they have been seeking avenues to pick up business, a merger being one of them:

Both Sirius and XM have been battling it out for the last several years and in 2006, both companies saw their revenues drop as well as subscriber numbers drop. This peaked a notion in the industry that it was very possible that the two companies were in negotiations to go through a merger.
The FCC has a good point, whether or not the companies are faltering as-is. Then again, what good is having two providers if there's no market for the product. Satellite radio has been slow to take off, and faces even more challenges with media players now being so easy to integrate with cars.