I don't think they sticky unofficial mods.

Probably because that could be construed as official support for an unofficial mod.

A case could be made for several mods being stickied (city mod, ai balancing mod, first cohort, etc.), but those haven't been stickied either.

Hell, MAA is a moderator with an unofficial mod, and his mod isn't stickied.

I think we're just doomed to search for things....but YOU can sticky any thread for your own personal use (just in case you weren't aware of that option, it's proven very useful for me). :)