Question

Why is Heracross OU and Mega Heracross BL?

Answer 1

Smogon's (showdown!) tiers are based off of use. People like heracross as a suspected mega instead of a regular mega I guess. The reason that regular heracross is OU and it's mega is BL is, like I said before, Smogon's tiers are based off of how much people use that Pokemon.

Comments

Basically, its because (1) some people use non-mega Heracross, making the total uses of Heracross greater than the total uses of Mega Heracross, and (2) mega-evolving Heracross uses up your mega evolution for the battle, and there are better megas out there.

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To be honest it's really because regular Heracross is better. It has better abilities, better speed, and it frees up your mega slot for something more useful like Charizard/ Venusaur/ Scizor.