Why are UMP45 and Intervention so common in S&D in MW2?

I never used to play anything in MW2 except Team Deathmatch, so I recently started trying out other game modes. And aside from less noobyness and the much higher percent of people with 10/other crazy high prestiges is Search and Destroy, I noticed that a LOT of people were using Intervention sniper rifles and UMP45's. Why is that, when most ault rifles are better then the UMP and the Barrett is obviously better than the Intervention?