There's a difference in the way the stylus development kits work. The Wacom update was done in seconds by replacing the older SDK with the newer one and compiling. When Adonit updated from their original stylus we would have had to completely remove the code that was added to support it and replace it with an entirely new approach that was too significant a change for us to manage at that time (also, there were questions about what would happen to users of the original stylus if we updated). Since then, changes may have been minor but we have delayed tearing the old system out until our next big iPad update which is currently being worked on.
So - We will be supporting the new styluses when we release our big update, small updates in the meantime have to be limited to stylus SDKs that do not require re-tooling input handling in the main app code. There's certainly a desire to do it, we've just not been able to practically schedule the time until our next major revision.