There has been lot of speculation on the internet poker forums about why Durrrr, who is also known by his real name Tom Dwan, isn’t a Full Tilt Poker red pro.
To give you short answer to that question: They don’t pay him enough.
If you want to get a longer explanation please keep reading.

Durrrr is probably the best known online poker player and thus in a sense would be a nice addition to FTP’s stable of poker pros. However from Full Tilt Poker’s standpoint everyone who has heard about Durrrr is already playing at Full Tilt Poker. Thus FTP wont offer him a great deal before he becomes a more household name among the people who don’t already play online poker.
It is quite obvious that Tom has been trying to achieve that type of status: in the last year he has began playing more live poker tournaments, has played on several poker TV-shows and generally has tried to generate buzz around him so that the poker magazines get interested about him.

Also it is important to note that because Durrrr is so successfully and has made so much money from poker he has no real reason to take a bad deal from FTP. More than likely he has been offered the standard FTP deal, which includes $35 for each hour the poker pro plays at FTP and bonus if the player gets television coverage. But as a poker player Tom thinks that he has better expectation in holding out and trying to become the household name and then signing a great deal, than in signing a guaranteed deal now.

Could Tom Dwan sign with other online poker site than Full tilt Poker?

In theory yes. However as an Americana player his options are somewhat limited. The only bigger American poker sites besides FTP that are willing to sign a bigger sponsor deal are PokerStars and Ultimatebet, and possibly Bodog poker. However there are big problems involved with signing with either room.
PokerStars demands that their sponsored poker pros play exclusively at PokerStars, thus Tom Dwan would miss out on all the juicy high stakes action that takes place on other poker sites.

Ultimatebet on the other hand was involved in a major online poker cheating scandal just a while ago. Because of that there is a very bad stigma attached to signing with Ultimatebet, and most likely it would lead to major dent in Tom Dwan’s reputation.
Bodog could be a wild card. They are currently sponsoring four poker pros: David Williams, Evelyn Ng, Jean Robert Bellande and Justin Bonomo. Having a young super high stakes online poker player in their stable would be perfect for Bodog’s image. However it is a whole another question if they are able to pay the type of cash that Durrrr is looking for.