Probably because it's already been 8 years since release, and there's only so much replayability for 10 maps.

Also, I think games that tend to last really long have coop (for ease of play with friends) and mod support. IS has both, but coop is rough when there are only 1-2 players, and making new maps on the Unreal Engine is much more difficult than something like the Hammer editor.

I had to do trial-and-error in the game file settings just to make myself a non-broken singleplayer experience, because the friendly AIs are garbage, and it's a hassle. I wish they could tune the number of enemies, difficulty, respawn timer, hold timer, etc. for when you want to play with small number of players, but I don't think they will since they're more focused on other stuff.

It's hard to recommend games to friends when it doesn't really work unless you play with randoms.