Or maybe MP only games just don't sell that good when compared to games with SP. You now, like every other MP only game and like a few of us mentioned earlier in this thread only to get ridiculed. I'm not saying they don't sell a lot of units but, when was the last time one was a run-a-way smash hit. Even BF4, a game built mainly for MP, has a SP campaign; no matter how dumb it may be, it's there.
Simple fact is that SP is still much more important to many gamers (I'm one of them), which is why not everyone who buys these consoles even subscribes to XBL or PSN. I'm not the best example because I use GF so much these days but, before I had GF (and since), I had never bought a MP only game but I have bought plenty of SP only games.