For 3 main reason:
1. Charizard and mega Charizard learn exactly the same moves. (obviously, since they're the same Pokemon)
2. MChar X can beat Dragon types easier with STAB and Tough Claws, and MCharY can beat Water types with Drought+Solar Beam. (so basically because they get better abilities)
3.Their stats are obviously way better, CharizardY having a massivbe SpA stat of 159, and CharizardX having high SpA and Att stats of 130.
in conclusion, the higher stats and better abilities make the Mega Charizards alot better than Life Orb Charizard, and can definetly defeat all the Pokemon it can, and more.