Because it takes sooo forgetiiiing looong
Bug types evolve faster than other types i think, also, all it takes is some Super Training to get those stats a-goin' (optional), fighting wild Pokemon around it's level, or using the EXP Share if you want to fight much stronger Pokemon/trainers if you don't want to just continuously use your low level Scatterbug on weaker Pokemon
It barely takes a half of a day to evolve it all the way to a Vivillon