I think it’s just the internet being the internet. Or at least how it’s been for awhile. There are big sites that a lot of people crowd to and that becomes the default. Like auctioning things off online. Ebay. That was where everyone went to. Need to order a few different things online? Amazon. Are there other online stores? Plenty. But Amazon is seen as cheap and convenient.
Nexus mods is just the popular site, but the moders have other options.