Before TNG there was only TOS (and TAS and the films) and that was Star Trek. While TNG was in its run I think it did get the lion's share of attention largely because it was the newest material and everybody seemed to be watching it. Also during the mid '80s to early '90s it was somewhat cool or at least okay to be into Trek since it was tapping into the mainstream.
But after TNG ended its run the broad interest didn't really translate to the subsequent spinoffs and films. Trek seemed to go somewhat off the radar for the broader public.
Although there are a lot of TNG fans I don't think it percolated into the public mainstream the way TOS did, which after all had a twenty year head start and it was the first.
The issue isn't which one has more fans, but which one has become and remained the most recognized. Before TNG we also saw mainstream science occasionally invoking references to TOS. When TNG was in production many of those references were transferred to TNG. But after TNG the references began refering back to TOS again.
TOS did have its commercial considerations as did TNG and the later shows. But perhaps it has something more to do with when the shows were produced. TOS came out of a time of creative innovation and experimentation in many art forms. The '80s had its measure of creativity as well, but the era is not generally remembered as such. And some of that sensibility may or may not have seeped into the shows themselves.
As the saying goes, absence makes the heart grow fonder....when Trek was gone for 10 years in the 70s, there was a huge groundswell of support to bring it back....STNG never had that. Its the only thing missing from the phenomenon equation of both shows. What happened instead was a movie immediately following and 3 other shows!
RAMA