preaching to the choir. So, quick history lesson that I'm sure you already know.
You know why Star Trek was so iconic? Because it was EVERYWHERE. It wasn't popular in primetime, but it was great for syndication... and it was in colour when a lot of other options were in black and white. There were only 80 episodes (instead of the usual 100 required for syndication), so everyone saw the same iconic moments over and over again. The green alien? That's only in three episodes. Just three, and Kirk isn't even in one of them. Klingons are probably only in a dozen episodes, perhaps, Romulans in two or three. But you just saw them over and over again, so it got burned into your mind.
So in the 70s, Paramount decided it wanted to make a fourth television channel and a new Star Trek was going to be the centerpiece program (sound familiar?). They started writing scripts, they built sets, they were all ready to go. And then STAR WARS came out, and Paramount realized "oh crap, let's do that instead!" So Phase II became The Motion Picture (which is why there's that dude who appears and all of a sudden dies, he was Spock's replacement because Nimoy didn't want to do a new series).
The first movie was in theaters for a YEAR and it made bank. The others did pretty good, too.
So after four movies and a 20th anniversary approaching, Paramount decided to make a new Trek series. But none of the networks were really biting. so instead they made the show direct for syndication. They'd make the show, and any TV station could buy it. Fox was relatively new, so a lot of the affiliates bought the show (and advertisers seemed to like the show because it was new content, a popular property AND family friendly without being "a kid's show"). So you could go anywhere and odds are you'd find Trek on TV.
Notice a pattern emerging here? Trek stayed popular because it was so ubiquitous and easy to find. Like McDonald's, except good for you.
Then in 1995 UPN decided to make its TV station and Star Trek was going to be it's flagship program again. They created Voyager. Now, Voyager was FINE, but it was a new channel you weren't instinctively flipping to, AND there wasn't anything else quite like it on the channel, in fact, the other programming seemed almost counter to Trek. Plus, now they had to fight cable and satellite TV. So numbers began to drop off. And when Voyager ended, they decided to make another sequel, when fan fatigue was really starting. Enterprise was the first one to be cancelled, after only four years, and it was stuck in this mid-space between wanting to be Trek and not wanting to be Trek. And at this time, sci-fi fans had Farscape, Stargate, and the new Battlestar Galactica to slick their thirst. While that was happening, the TNG movies weren't setting the box office on fire because they became dumb action movies, very un-Trk-like at the core, alienating the fanbase.
So now we're seeing that happen again, making a Star Trek that's sort of Star Trek but not really, locked behind a paywall when there's other fulfilling content that's easier to access (Dark Matter, The Expanse, Killjoys, Legends Of Tomorrow, Doctor Who, The Orville). So why would anyone fork over money for an app for that one show? On top of that, Discovery is on NETFLIX in every other country in the world. So it feels as if the entire American fanbase is being asked to take on an unfair burden with CBS All Access.
You know what Star Trek was like in the 90s? It was the Marvel Cinematic Universe, on TV. It really was. You had this universe that expanded three TV shows, with characters popping up in each other's series. There were comics and novels, they even made a new "series" in book form called "Excalibur" that was pretty much presumed to be canon. But the key factor was that it was easily accessible. It feels like Star Wars really stepped up to fill that niche in the last couple of years, with the movies and the games and the new burst of novels and comics.
TL;DR: Star Trek and CBS seem to be doing everything counter to what made Trek popular in the first place.