Quote from ridd3r.:
I think it's more that I just found everything in the game and it would be a waste to not put it to use! I've not got loads of free time, so I'd rather maximise one game. There're plenty of runners to run other games, so I think specialising makes sense.
Yeah if you've found one good game that you're not getting bored of, it's not such a wonder if you keep to that one. And MP is definitely popular enough so you know you're always gonna have people interested in what you've found.