Also, and I know that this is touchy to bring up, but the name VGMix just has the renown of being "that other VG remix site like OCR but not". People knew about it, and if a new site comes up at that address or with that exact name, I know at least a few people will go there based on name-alone. Functionality does factor into long-term popularity and usage, but just like OCR, the community of VGMix is at least half the experience and enjoyment. Not only was the community ridiculously active, but that instant-post/feedback feature was key in keeping people active. The control was in the hands of the artist, and they were more motivated to make things better, and quicker, because of not having to go through any kind of wait besides the regulation posting limit (and of course the one they put on themselves while they create the song).
Now I'm just starting to reminisce, heh. The cycle was a joy in-total: post a mix, get reviewed, factor in reviews to make next song better, and do it again. Talk to reviewers and get more specific feedback, maybe even start a collab, listen to other mixes, learn and review the mixes yourself. There was a very good flow of everything...
Halc nailed it on the nose in my opinion.