Vassal has been used to play over a thousand different games for a decade now. It'll take Tabletop Simulator a while to get anywhere near that much traction.
In a lot of ways the cruftiness of vassal is a filter function- if you're not willing to put in the effort to learn vassal, you're not likely to have the motivation to play the kind of complicated games for the amount of time it takes to reach completion that vassal was made for. An upsurge in popularity caused by something like a Steam sale would probably kill the community, or at least change it beyond recognition.
Take a look at its workshop page on Steam. It really does have a nice selection of games. (I think Vassal probably has more wargames though if that is what you are into)
