It's a few things.
Most obviously is nostalgia. You have to really try to break past Pokemon on that point, if the whole national dex controversy showed us anything it's that Pokemon's fanbase is incredibly loyal.
Another thing is Pokemon's concept, though really simple it's incredibly solid and well executed on, at least at base. A modern world that's pretty much ours except for the fact that there are magical animals and monsters that roam around and befriend humans is bound to get people engaged and interested. It's easier to recognize things and relate to characters and the world when it's so similar to your own.
And lastly the designs. No other monster collection game has designs that work as well as Pokemon's. That isn't to say other monster collection games have bad designs, but Pokemon's monster designs have some sort of elegance to them. I'd attribute it to the fact that a majority of Pokemon are designed with a purpose, they fit into an ecological role, and that helps feed the fantasy of "This is my world but with magic animals" more than anything. And this design philosophy of giving everything a purpose and a niche leads to interesting designs that make Pokemon really memorable. A lot of the TemTem I've seen kinda just look like fucking moeblobs with cute faces or cool aesthetics. I know it's a buzzword that's thrown around a lot, but it genuinely feels kinda soulless. Like they took inspiration from other monster collection titles but didn't think about the reasons they worked, the best TemTem was a fucking fakemon at one point for fucks sake, designed with that similar philosophy of "What does this do?".
