Why?

1. The fact that master is 5.2 is currently expressed through a branch alias, which wouldn't needed at all if the branch was named in the conventional way.
2. It is clearer to contributors what branch they are sending PRs to, rather than them mistaking master for latest stable (5.1) when it is actually the next upcoming minor series (5.2).
3. The word "master" has undesirable connotations, and GitHub itself has announced (or leaked to the press, or whatever...) that it is looking to replace this term for the default branch of repos, anyway.

Possibly, as an interim, the master branch could be left around for a few months, and synced with 5.2 (to allow people to migrate if they have master or dev-master hard coded in various places).

EDIT: One problem with 5.x as a branch name is composer will install the 5.x branch with the version constraint 5.7.* even though there is no such version. This is why 5.2 is better, because it matches the current branch alias of master.