Everything was explained at comments at news section
"#8 Xantos, 27 Mar 10 12:21
Hey there,
the choice of rooms is based on several factors:
1. PartyPoker, bet365 and Everest have been the top3 choices for new Free $50 players in 2010 so far - and we want to make sure most beginners can at least participate in one of the tournaments.
2. Poker rooms must allow us the tournament and make it technically possible to only allow players tracked to PokerStrategy.com - which is sadly not totally easy most of the time.
3. 888.com was the sponsor of the last BM party - and thanks to their Free $8 offer in February, a lot of players have an account there.
4. EuroPoker is our Ongame partner that is most interesting for experienced players & we want to encourage testing it out.
Additionally, we think that for experienced players it's an acceptable effort to create an account to play a big tournament. We agree that other poker rooms would have been more comfortable for other players - but there's always a need for compromise.
Lutz "