Is it just me? Here are the pictures:
There's not really a reason (other than Game Freak getting lazy), and probably wasn't intended to look that way, but they are sort of similar; you could say that. The colors are certainly similar, so I can see why you'd think that. Do keep in mind though this happens a lot; back when we didn't know much about Unova's new Pokemon, many of us believed that Alomomola was an evolution on the pathetic Luvdisc.
Only game freak knows why. They made it like that..... maybe they are cousins. Write a letter to game freak or something... I doubt they will reply.