G'day Jcooper
M.P is limited at altitude because due to thinner air up high, the throttle needs to open wider to draw the same amount of air molocules than for lower altitude denser air. The M.P is limited plainly because it comes to a point where throttle cannot open any further, so therefore no more M.P will be available, and RRPM will begin to decay. If you look at the M.P limit placard it will read wide open throttle at high temp alt combinations.
Hope this makes some sense
