TNG was the most popular show...
I think that's debatable. It had more viewers in first run but struggled in strip syndication and at the theater in comparison to TOS.
And TOS is more broadly known among the general population.
well i don't want to get into the bones of it, but it pulled the sort of numbers a T.V show needs to stay on the air these days, and right now that is more important than syndication, because lets face it, unless any new Star Trek pulls TNG numbers out of the gate, it won't get more than a season to put into syndication.