Why is docker unsupported for 32bit Linux systems?

I’m aware that docker is not officially supported yet for 32bit Linux systems.
However it seems to be possible to build it on a 32bit system on our own. (http://mwhiteley.com/linux-containers/2013/08/31/docker-on-i386.html).

I’m wondering why docker has not officially supported it yet. Is there a performance issue when running docker engine on 32bit systems, or maybe some other issue? Is building on a 32bit system on our own not advisable?