Is due to a couple of factors IMO:
First a predominant color help with the details, the lack of details is less noticiable...
Second is for the feeling, one game has success with a grey enviroment and then more games are grey, brown... etc... because colors are so last gen and childish... is just a trend... a bad trend...
BTW they are good enviroments with more colors, like Crysis, Bioshock or the Orange Box...








