What's the point of exp rates?

I'm playing through White2 right now and I'm trying to raise a Swinub. Around level 20~ I noticed how abysmally slow this thing leveled, not to mention how weak it is before evolving. I don't understand why I have to grind more to raise this guy when my other Pokes, Dewott and Lucario, easily level.

I can understand having it on Legendaries since they have a single evolution...but it's making the game a hell lot more boring because he dies easily pre-evo and levels so slowly.