If runnan can crit, why cant hydra?

You want an AD/Lifesteal/Waveclear item to Crit as well? Such greed :3

Because it already has waveclear, lifesteal and AD. Why should it crit?

If it did crit then only melee crit champions (Yasuo, Yi, and Xin) would really benefit off of it. The main crit class (marksman) couldn't use it.

We have enough crit items. We don't need more.

That's not why they don't build crit. They don't build crit because all of them get more from other mechanics,. generally because they have higher base damages and do a higher percentage of their damage through abilities than ranged champions do, and they can't afford to itemize into the pure damage and attack speed that crit requires to be most effective.

The reason Hydra doesn't crit is because it doesn't need a special synergy with the very few melee champions who might want to build crit. It doesn't need to have anything balancing it around them.