Because it evolves at Level 59. Gamefreak made it like that.
A possible theory is that Volcarona is quite powerful, and could be overpowered ingame if it evolved earlier.
How to change your username How to add a user avatar Why was my post hidden? Question etiquette Flags vs Downvotes Anime FAQ Official PokemonDb League
Type a Pokemon name, type, move or ability to see instant results.