Why can't he finish inside?
What the heck?
From my view it looks like he just doesn't have the touch that most guards do. He front rims a lot of floaters, doesn't use the glass well, and doesn't have great body control in the air.
It also looks like he gets trapped between wanting to create space for a runner/floater and wanting to create contact to draw a foul. I think this comes from the fact that he rarely gets any calls, largely because of how he plays defense. He is never going to get the respect that IT got, for example.
I would like to see him improve his off-hand and also shoot more one-handed shots. I think Kyrie could do him wonders by just showing him a couple up and under moves to work on. Marcus seems like the type of guy who has too much pride to want help but he needs it on offense. The ball is simply not going through the basket for him.
That being said, I don't really think he cares too much about any of this. He would rather win games, box out, and fight 7-footers.