Red Dead Redemption 2 suffered another delay recently, but the popularity of the upcoming game and the hype surrounding it isn't dying down. In fact, seeing so much excitement for the next big thing from Rockstar Games, it's easy to assume that when the game drops, Grand Theft Auto Online will bleed players like crazy.
Thing is, that very likely won't happen, and there are a number of factors contributing to this.
First of all, people need to understand the fundamental difference between the nature of popularity the two games enjoy, and how that popularity differs. If you look at forums, subreddits and other online spaces where gamers discuss their hobby, you'll likely see major buzz around Red Dead Redemption 2, talk of possible features, story elements and other speculation. The same was true for Grand Theft Auto 5 prior to launch, though on a much bigger scale.
The underlying difference between the two games lies in their audiences. GTA 5 is much more mainstream than Red Dead Redemption 2 is, and the gamers who go online to discuss their favored titles represent different ratios of the two games' player bases. The "hardcore" GTA Online players you'd see active in the subreddits devoted to the game represent maybe ~10% of the whole audience, for a more "dedicated" gamer oriented title like Red Dead Redemption 2, this vocal core gaming community represents maybe as much as half. And even then, the GTA 5 community is a lot livelier.

"Sure, but Red Dead Redemption 2 hasn't even launched yet, of course there will be a bigger community after release" you say, and though you are right, keep in mind that the basis of comparison is a game older than four years. Typically, the period of time prior to a game's release attracts a much bigger community than a several-year-old game does precisely due to the pre-launch hype and speculation. At this point, if Red Dead Redemption 2 was going to so much as match the popularity of GTA 5, it should have a larger active fan community — but it does not.
The crux of the comparison is that GTA 5 is everyone's game.
Looking at the numbers, with over 90 million copies sold, almost everyone who owns a console also owns GTA 5. It's the most mainstream video game out there, on par or even eclipsing the likes of FIFA and Call of Duty.
Red Dead Redemption, though highly popular, is basically niche compared to GTA, with a much smaller audience comprised not primarily of casual players but dedicated gamers.
Simply put, Red Dead Redemption just doesn't currently have the kind of mainstream clout and reach that GTA boasts due to its long history, the snowball effect of its sweeping popularity and the consistent growth granted by free content updates. Over time, Red Dead Redemption 2 and Red Dead Online will also grow and grow, but it would truly be an amazing feat were the game able to pull off the industry defying stunt that GTA Online did and still be growing after 4 years.