I can't speak to the original intentions of the authors, but I suspect it might be related to difficulty of making significant changes to the project due to some strongly held beliefs.
An example: react-native depends on alpha/beta versions of react, and libraries in the react-native ecosystem tend to include a peer dependency on react (eg: react >= 15). React 16.0.0-alpha.12 will not satisfy this range, but 16.0.0 will. It's unclear to me in what way it is useful to exclude pre-release versions, and this causes a lot of confusion for users. I posted about it here: https://github.com/npm/npm/issues/8854 and it was shutdown for ideological reasons, rather than practical considerations. On the other hand, someone submitted a patch to yarn to improve this behaviour and it was quickly accepted: https://github.com/yarnpkg/yarn/pull/3361.
An example: react-native depends on alpha/beta versions of react, and libraries in the react-native ecosystem tend to include a peer dependency on react (eg: react >= 15). React 16.0.0-alpha.12 will not satisfy this range, but 16.0.0 will. It's unclear to me in what way it is useful to exclude pre-release versions, and this causes a lot of confusion for users. I posted about it here: https://github.com/npm/npm/issues/8854 and it was shutdown for ideological reasons, rather than practical considerations. On the other hand, someone submitted a patch to yarn to improve this behaviour and it was quickly accepted: https://github.com/yarnpkg/yarn/pull/3361.