Why would you release extra content for a game that nobody, critic or consumer, actually liked? The best I've heard about Colonial Marines is that it's not as awful as some people said, it's just pretty shitty. I mean, it could be a complete overhaul of the guns and the combat system and the sound effects and the level design, with better optimization to fix the loading times and improved AI and free steak and orgasms for a month because why not, but otherwise, who's going to buy it?
I still haven't played BL2. I'm waiting for it to go on massive sale ($15 or less, preferably less). It looks neat, and Poker Night 2 unlocked some stuff for it, but I'm not paying $40 for it, plus full price DLC.