Well it's not like J Mascis just disappeared, or that the Dinosaur songs have been locked in a vault. He still tours, a lot, as J Mascis & The Fog, and they play more Dino than Fog songs.
In fact, at the last Fog show a month or two ago at the Black cat (which was great), the only t-shirts they had for sale were Dino Jr. ones. Which were very cool, but $25?