Charizard is not powerful enough to be in OU but it still is in OU because of its Megas.
Charizard is in OU because it is used extremely often. (Hence the tier name; OverUsed). It is used often solely for the purpose of mega evolving it, thus making both of it's mega evolutions OU.
Alakazam however is not used quite as often, but Mega Alakazam is just too powerful to be in the UU tier. It would simply destroy most other things in the tier with almost no threats or counters. So the reason for the tier difference between Alakazam and Mega Alakzam is because Alakazam is not used often enough to be in OU and is not a large enough threat in UU to be placed in BL like its mega evolved counterpart.
It's because Smogon tiers are based on usage of Pokemon. Alakazam's usage lands it in the UU tier, but it's Mega was deemed to powerful for UU, so it's in BL. On the other hand, Charizard's usage puts it in the OU tier, and it's Mega is not too OP for OU.