Shortest answer (one of a few reasons): limited user account permissions. But it is something that we'd like to include in a future update.

FYI, maps created using the in-game mapmaker do get transferred; only Galaxy Forge maps do not.

As a custom map maker, I just wanted to chime in to add my support for patching the game to allow auto-downloads. The map files and the .tga files (the title-picture boxes you see in the game setup screen) are very small and should easily and quickly transfer between hosts and players for this game (unlike much larger and sometimes gargantuan first person shooter maps).

Yea totally agree. As a custom map maker myself , I find it difficult to convince people to download custom maps in Sins . There is alot of (CBA) "Cant be arsed" syndrome and I dont blame people for it. In other games like Homeworld 2 , you just have to join a game to get the map. In Supreme Commander you can download maps in lobby or as you join a game. In SW empire at war , its auto-downloadable upon joining a game.

Alt-tabbing out, restarting sins, finding the right map to download , finding the correct folder ,unzipping...all contributes to CBA.

Really hope this comes out with 1.1

I also dont see why you cant have save-games being transferable , so that if someone de-syncs you can dont have to email them the save-game.

we will see, i belive we can se some kind of auto download later on maybe in patch 1.1 or maybe in the expansion.

The thing i wait for is a few tournament maps that will be the same for all...

I going to try to pimp Ring of Risk some on ICO and see if I can get some folks to download it and add it to their game. I really don't think that having to spend the three minutes needed to do that is too much for someone who takes the game seriously and who's getting tired of playing Maelstrom and the not-so-random "random" maps.

Heck, back when I was playing the original Unreal Tournament on dialup I would wait over 5 minutes or more just to download a new custom map and I would find out what new maps had been added to servers and then download them outside of the game, unzip them, and install them. I would hope that Sins players would be equally enthusiastic about Sins.