Why isn't FCM open to both factions?

But I did free character migrate my alliance mage from Kazzak, where alliance are dead, from the last FCM (EDIT: not sure if it was the last FCM, but it wasn't long time ago).

Also,

Alliance
Outland -> Darksorrow
Silvermoon -> Doomhammer

Alliance are given FCM to alliance dominated servers? I don't understand.

Darksorrow and Doomhammer aren't Alliance dominated.

Edit: Clarification.

At first I thought it's about faction balance too, but then we have those 2 transfers that doesn't make sense. Though my main question is why they discontinued giving both factions FCM if there is a queue issue.

We're getting into territory that's already being discussed in another active thread. You should check that out and continue your discussion there :)

Nope, both Darksorrow and Doomhammer have more alliance, the balance is very bad in Darksorrow actually.

Based on the actual realm data I'm looking at right now, I can safely assure you that is not the case :)