Millions is a overstatement, there are some hundred thousand people who enjoy NES til this day still. It's a classic and part of the childhood of people who started playing games in the 70's and 80's. While kids from the 90's don't have nearly as much love for 64, Saturn and PS1 era.
I actually think this is much more a difference in culture and how games are consumed for different age audiences. I'm a kid of the 90's and I have no interest whatsoever for either PS classic or NES classic
If you ask me whether I like more PS1 or NES, then I would easily say PS1. I would like to play some of their games if they were available on some cheap subscription service, but would never buy a Console specifically made to play them
I don't think it is though. The NES Classic sold 3.6 million units as of june 30th 2018. It was regularly out of stock at retail while it was in production, and heavily scalped on the secondary market. Nintendo could have easily sold quite a bit more if they were interested in maintaining production, but by September 2018, Nintendo was pushing the NES games as a perk of Nintendo Switch Online paid subscription.