I believe the reason is that they just hadn't gotten around to it yet. They were working on upgrading the swamp and the mountains, and then everyone got antsy about not having a new biome, so plains was left behind while they concentrated on mistlands. And from the mistland interview video, it looks like the plains revamp will take a very long time as they put ashlands ahead of it.

Originally posted by Tangoblue:

Do you really want to run around the Plains in light armor?

The root chestpiece is a very good thing to wear in the plains already. The pierce resist works great against deathsquitos and spear throwing fulings. And if you're not planning to raid a village, putting on a piece of fenris armor will keep you from walking like a turtle. In some cases, the tradeoffs are worth it.