As simple as that sounds. Getting creatures to level 30 is already a grindy task to do (wheter be because of DNA or coins), and a new rarity would imply doing that, even to players that aren't that focused in the competitive aspect, and only want to create new animals.
Closest parallel I could think of is Pokémon. While they have evolutions, they do not keep evolving constantly until level 100. Most of them are already fully evolved way before even reaching half of that level. This game should be working like that. If someone wants to level up to 30 a new creature by themselves, then no problem, but if someone has to do that effort to fuse for a new creature because they think they look cool/cute/good, then that's already a problem that would make a huge chunk of the remaining playerbase just go away.
We do not need a new rarity.