Bug#220493: So why is Debian's XFree86 old?....
The short answer is "porting". The XFree86 project has little or no
support for most of Debian's architectures, and making sure that a new
version works on non-i386 machines takes quite a while.
The second answer is "integration". The number of packaging issues
which come up in packaging such a big project are very large, and it can
take quite a while to resolve them all.
The more specific-to-right-now answer is that even adapting all of the
preexisting Debian patches (against 4.2.1) to patch 4.3.0 seems to be
taking quite a while. I believe that the way they are being updated is
intended to make this process easier in the future, although any member
of the X Strike Force may correct me. :-)