I think, "running out of types to give Eevee," is enough criteria. The only thing for me is that all Eevee types look generic. Dark type is black, Fire is red, Water is blue, Electric is Yellow etc
If they wanted to make a Flying type they would have to drastically alter Eevees look, the same could be said for Fighting and Dragon as well but not, I don't think, Steel, or say, Ground. "Make them silver and brown," is all the thought process they usually need.
As for a normal type it would be like, "Okay so we found out Eevee could evolve into x different types before we realised it naturally evolves into the same typing." No.
If they wanted to make a Flying type they would have to drastically alter Eevees look, the same could be said for Fighting and Dragon as well but not, I don't think, Steel, or say, Ground. "Make them silver and brown," is all the thought process they usually need.
As for a normal type it would be like, "Okay so we found out Eevee could evolve into x different types before we realised it naturally evolves into the same typing." No.