Because there's no real problem with having a 1.5MB glibc on a server or desktop OS, and glibc is quite a lot more featureful than the alternatives. Many lack locale support, nsswitch, backwards binary compatibility and various minor bits and pieces that breaks existing applications.

The size isn't really an issue for most desktop or server oriented distributions. Perhaps bigger issues for distros are:

- Symbol versioning: If you have a newer glibc installed when you build your packages, they'll refuse to run on older glibc, even if they don't need any of the new version's features. This makes it hard to make packages that support multiple versions of a distribution (e.g. a stable release with an older glibc and an unstable release with a newer one).

- Awkwardness of upgrading: For example Debian usually wants you to restart X and any daemons that use nss when glibc is updated. There are also atomicity issues during upgrade where there's a race window during which execing programs can fail.

On the other hand, distros are invested in a huge set of packages as well as support for third-party (often non-free) software, all of which are presently linked with glibc. I think musl is a lot more interesting right now to new and upcoming distros that are looking for a fresh start with less baggage than as a migration path for the big distros with major user/customer bases, at least in the short term. In the long term, who knows what will happen... :-)