Well, opinions seem to vary on why glibc was originally forked by the linux developers, but it's ancient history now and doesn't matter. What seems to be the case is that glibc continued to be developed (largely by the much-maligned Ulrich Drepper) and got some very desirable features (here is a list), and the Linux devs at some point decided it is better to drop libc5 (as linux-libc was called) and switch to glibc 2.0. Red Hat was the first, followed by Debian.
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