The simple answer is, "There is none."
For some reason, year after year, I continue to read things from fans about how so and so "isn't deserving." My question always is, "How can they not be deserving if there is literally no criteria?"
Please stop trying to apply your own criteria to the WWE Hall of Fame. In addition to that, please stop asking why so and so isn't in yet because there's clearly a template to each HOF class, and you can't put everyone in all at once.
Could Rick Rude have gone in before now? Sure. Should it bother you that he is just now going in? Absolutely not.