Generically, people are lazy. If they see some mod: "Oh, I don't have it". I bet you, at least 75% can't be bothered (or do not see them at all due to the DLC filter) , they are happy enough with stock content as there are enough races in there or they are not capable of installing mods. Maybe like 5% of the viewers want to get the download, they open one or two of their favorite sites, can't find it and quit. Those who never visited mod sites will have an even harder time finding the correct AC mod through search engines, as most mod hosts do not show up in the first page. (Think of 'mod-blogs')
I bet you, that out of 100 server name 'views', 1 or 2 will actually download the mod and join the server. Marketing is not that easy to just 'host a server with a mod' and downloads will flow in.
Besides that, how do you want to get your server viewed in the first place? Top spot servernames are generally taken, DLC filter exists and a full server will not be a feasible 'marketing' method as you don't have that big a publicity for a specific mod.
@D1visor is right stating that the Workshop would generate a lot more publicity, ease-of-use and automatic updates to keep people up-to-date and constantly able to use the server.
As long as there is no Steam Workshop, we provide our services to enable modders to find a bigger target group through our '
Downloads'. But even then, not everybody who owns AC, knows RD.