why doesn't 0ad want steam release?  

So, uploaded DE r28 to mod.io tonight. It just needs to be approved. Thank you to all of the playtesters. There are still several things I could fix and improve for r28, but at some point you gotta move on to the next development cycle. Here's something I'm thinking of doing for Release 29 ( thanks to one of @vladislavbelov's latest pull requests):    

Honestly, I didn't know that was the case; manually updating the game is tedious. Besides, I thought the Steam Workshop would add quite a few mods to the game.

I have a feeling steam wants 0AD on their platform, I doubt these recurrent posts are spontaneous gamers registering to make so nicely written comments, explaining the benefits it would have for an open source project. 

Being open source doesn't prevent you from publishing the game on Steam. Why not release it for free? Releasing it as early access on Steam would provide bug feedback and allow many volunteer developers to contribute to its development.