How come Maokai doesnt have mr per level?

Because he was pretty op till nerfs would be a nice buff.

Because of his ultimate. If you look his hp per level is also poor (for melees) due to his % damage reduction.

He was designed and released in a time when tanks were very, very strong against mages, particularly tanks with notable defensive steroids like him. MR and health options were stronger, and mages tended to be more burst-reliant. Maokai was designed without MR/level as a way of limiting his potential strength against them. Then the game changed around him, but he never really needed the MR/level (at least, not as much as he needed the other buffs he got and the change to his ult) so he never got it. If he ever needs buffs, giving him the MR/level would probably be a decent option.