The reason is that David Anders is a "regular" according to ABC, while Lena Olin as Irina is a "recurring guest star". Yes, there has been more to talk about Irina-wise recently, and I would create an Irina forum, but she's only signed up for 13 episodes - which may mean she is leaving soon unless her contract is extended.
But if she is still talked about in a few episodes, a Lena Olin forum may be created. The reasons there is an Irina forum is...well... I didn't create it do I don't know.