I am just a poor man from poor 3rd world country called Ukraine (ex-USSR). I'm not the very smart person and definitely is not the best Python programmer.
I started hacking on bzr in August 2005.
I was de-facto maintainer of Bazaar version for OS Windows (2006-2008) and built installers for this OS. But too long I saw like many core developers constantly wrote windows-hostile code and nobody actually care about windows support. The most stupid thing is dirstate working tree format with OS locks. This is the real pain in the ass for Windows support and also it tends to fail horribly on NFS (Linux) and AFP (Mac) as well. There is also many medium and small things that was missed in original design of bzr that makes life of windows bzr users much harder than it should be, like: case-insensitive filesystem support, cross-platform line-endings support, cross-platform support for symlinks. (The biggest thing missed in original design of bzr is performance, of course).
Of course all core developers are Linux guys and get paid from Canonical Ltd and therefore windows support is the lowest priority here (to get bright picture, see bug #1: https://bugs.launchpad.net/ubuntu/+bug/1). And because I need to work for money I don't have time or desire to support bzr in my free time anymore.
If you want to be next windows maintainer all you need to know about windows installers described on the page BzrWin32Installer. But as I can see there is nobody who want to help. Although I promised that I will continue to build windows installers until new maintainer replace me, but I don't promised I will do this forever.