I'd guess it's because it's a completely different architecture, eInk is such a niche use that the engineering resources needed to provide an SDK probably outwe

I'd guess it's because it's a completely different architecture, eInk is such a niche use that the engineering resources needed to provide an SDK probably outweighs the development community that would make use of it. The Nook Color on the other hand has a huge development base and the SDK heavy lifting has been done already via the underlying Android platform.