There is no basketball reason why Marvin plays over Carroll. It's not even defensible.
My personal guess is they don't want to completely obliterate Marvin as a possible trade asset. And taking him out of the rotation entirely, which is the right basketball move, would basically guarantee he's a dead contract.
Warning: this post occasionally contains strong language (which may be unsuitable for children), unusual humor (which may be unsuitable for adults), and advanced mathematics (which may be unsuitable for liberal-arts majors).
Stolen with love from the XKCD comic.
Marvin is a solid defender, pretty ho hum on offense. He's not terrible, he's just terrible for the position he's been given (free pass, starter).
We need to play the best players. Carroll, Burks, and Hayward give us more than Marvin imo.
We pay Marvin too much to not let his see the floor. And his small amount of trade value he has would cease to exist.