There is quite a strong incentive. What there is not, is means — Forge and Minecraft have both changed dramatically between versions, to the point that many mod developers throw their hands up in the air and rewrite the mod entirely. For something complex enough that that isn't an option, for instance Electrical Age, it's easy to remain stuck on an older version forever.
There's no documentation, and the API owners often assume that forcing a complete rewrite of major parts of the mod is fine. It's really not.
