When you say "Star Trek" to the average person in the U.S., the first thing that probably pops into their mind is something like this:
Instead of this:
That's not to say that TNG wasn't popular among science fiction TV fans in the U.S. -- it was hugely popular. However, the knowledge of who Kirk and Spock are among Americans goes way beyond just science fiction fans. People who are NOT
fans of the genre (people who don't watch Star Trek TV shows, or science fiction in general) still know those TOS characters, while many of them probably have no idea whatsoever who Data is.