I can't speak for cart here, but from my perspective as a contributor to `bevy`, it's extremely useful having freedom from backwards compatibility concerns to improve our APIs.
For example, we want to rework our input handling, asset handling and UI; supporting this current UI solution indefinitely would be frustrating when we hope to make something much better once we can focus on that. As an example of this, we massively changed our rendering APIs for this release, allowing the improvements listed in the blog post to be implemented.
I wondered why the Rust community seemed so averse to 1.0 releases, but Rust for Rustaceans makes the interesting point that there are a lot of cases in the Rust language where adding new features technically qualifies as a breaking change. There doesn't seem to be a lot of room for the minor version number - everything is either a revision or a major release, and people aren't ready to commit to that level of strictness until they're feature-complete.
For example, we want to rework our input handling, asset handling and UI; supporting this current UI solution indefinitely would be frustrating when we hope to make something much better once we can focus on that. As an example of this, we massively changed our rendering APIs for this release, allowing the improvements listed in the blog post to be implemented.