That's an excellent question. swift-testing is an experimental package that relies on language and toolchain features that aren't available in Swift 5.9. The changes that we've made to Swift Package Manager to experimentally support swift-testing are not available in Swift 5.10 or earlier.
We use semantic versioning with our releases. Releases prior to 1.0 should be considered "unstable" with all aspects subject to change. In the case of swift-testing, this includes the minimum Swift version needed to use it.
We continue to include basic support for Swift 5.10 at this time because we recognize that not all developers are able to switch to Swift's unstable main branch just to use swift-testing. There are features in Swift 5.11 such as SwiftPM's GitInformation structure and the proposed low-level linkage control that we are adopting or looking to adopt. Because we'll be relying on these features, a minimum Swift version of 5.11 will be necessary in the near future.