It was a temporary mission, similar to the Deep Space Nine missions that were released with the Dominion Story Arc for a short time. They were there during the featured period and then Cryptic pulled them out of the queue list. I'm guessing the Elachi Alert is gone for a while at least, they could bring it back, but probably not in the near future.
I read more into the post I linked above, and it looks like they have some "plans." You're probably right about this being a temporary mission. Darn, it was the only space mission worth doing if grinding Romulan marks.
Guess it's back to that 20 man ground mission. How disappointing.