The bizarre reason being that tex.el should be checked in as a binary
because of CVS version strings. It's not like this has not been
discussed already. AUCTeX 11.81 uses a different strategy to generate
version numbers and will probably work without such measures, at least
when built from the official release.
Of course, XEmacs central won't accept binary packages that have been
built with anything but their personal infrastructure. I am
contemplating offering prebuilt binary packages from the official
AUCTeX site (complete targets for that are in the Makefile), but on
the other hand it seems pointless to go out of one's way to promote a
product which makes a point of making cooperation difficult.
If XEmacs central prefers a modus of operation which tends to lead to
outdated packages not maintained by the core package authors, then we
probably should respect that choice.