are there any hurdles or other good reasons to not just adding this to every create?
I'm no expert. But my guess would be that many crate authors may simply not be aware of this feature. It wasn't always there, and it's still unstable. You would have to reach the "Unstable features" page of the rustdoc book to know about it.
Or maybe some know about it, but don't want to use an unstable feature, or are just waiting for it to possibly automatically work without any modifications.
Of course, I would assume none of this applies to the embassy devs. That Cargo.toml file has a flavors field, which is something I've never seen before 😉 So, I'm assuming they are way more knowledgable (and up-to-date) about the Rust ecosystem than me.