Why Zilean and Volibear are rivals ?

Lol, it's not really an ingame thingy (atleast not yet, unless they update zileans lore) It was cuz the designer of Zilean jokingly said that league wouldn't have armored bears, so when volibear was released it became a running gag/joke ("bish u thought" sorta thingy xD)

The rioter zilean was named after was opposed to the concept of having an armoured bear as a champion, which produced the rivalry behind those champions.

I don't know if any subsequent lore was written to justify it in-universe.

As for zil balance, he's one of those tricky cases where he exists as both a solo laner and a support so extra care is needed to ensure buffs for one role don't break him in the other.

zilean hates armored bears