I think the main reason is that DWM is a very minimal windowmanager, it doesn’t even have a userland config file. To change it’s config you have to edit the C header file and recompile the runtime. So for that reason an official package wouldn’t work well, because you wouldn’t be able to recompile it easily.
More information: dwm - dynamic window manager | suckless.org software that sucks less
A few other tiling window managers like i3 are slightly less minimal and do offer userland configuration, so they will be more easily available in the official repositories.