The built-in #[derive(..)] macros have been there for quite a long time, and they have a specific set of semantics (their order doesn’t matter, they only add a new implementation for that trait, and they only work on specific types). Changing the semantics of #[derive(..)] now seems like a very bad thing to do, as it could just confuse users. Macros 1.1 was also designed to be a small api, so it could be stabilised more quickly.
If stable procedural macros are a desired feature, then maybe their priority should be increased.