Doesnt surprise me tbh - people are used to getting freebies off the net its todays culture, a lot of people may not have credit cards - and even if they are old enough probably dont want to explain to their other halves why they are spending money on a gaming site - when they can download most of it for free anyway.
The people who care have paid up - no surprise either - is always the way with a lot of hobbies/clubs where the few support the many - and the many have a good time - but the alternative is to have nothing.
If you want money you have to force the issue more - probably at the expense of more members joining etc for example You could get rid of free downloads and have a basic joining fee 1000 downloads for say $10 / £5 maybe.
Or you could put the best / most popular mods into an area that only paid up members can access.
Before you do that though I would suggest having a very big banner on the Home page - making it much clearer than it currently is that subscription is here and its purpose to support the site.