What about Alcatraz? Community absolutely loves that mode, and only few times that community unanimously agrees that Alcatraz should be permanent. Why it is a temporary mode?
Alcatraz! Well, first off it is back at the beginning of Season 1 and for two weeks straight. However, to your second question about it being temporary - while it is popular in this particular community it is not quite popular enough to keep as permanent. We think it works rather well as a limited time mode that tends to return every season.
We worry that if it were permanent it would become stale with a low-play rate, which will affect matchmaking for it, and in its current limited time form it remains relatively popular each time it comes back. That's essentially the thought process behind any mode being limited or permanent. Hope that makes sense, but if not feel free to ask more questions.