Why don't BMX plugs fit on MTB riser bar?
The outer diameter of bars is the same. Internal diameter can vary a bit between manufacturers, but you're going to have an ID of around 20 mm for road bars and 17 mm for mountain bars (source). A BMX bar is somewhere in between -- if you get the right MTB bar and the right BMX plugs, they'll probably work. 

That being said, if you can't get those in with a bit of twisting and turning, you could probably shave them off. Or, go for an expander style plug.