SS1 was bound to be less popular than the sequel when the sequel was actually appealing enough to play like a videogame, and many people have fond memories of it as a result. SS2 is compelling enough and complex enough as a game (and is so unlike anything that came after it) that it makes perfect sense that everyone loved it, even if it's flawed.
From memory SS2 barely eeked a profit and was not a big seller. It felt archaic and primitive compared to the newer shooters coming out. By contrast, SS1 blew people away.
Admittedly SS1 ran at something like 16fps and had a mouse-heavy interface in a very 90s way, and was a pain to actually get running like all pre-Windows Origin games. The interface and general playability were things you could hand to SS2 before the enhanced release.
It's funny to me because most of the things people attribute to the "immersive sim" genre SS2 lacks. It's quite linear and lacks alternate paths almost everywhere; you really have to complete the game in the same order every time. SS2 often just forces you to walk through gauntlets of enemies and gun turrets with little in the way of side routes or even cover, and primarily is focused on resource management. I view it more as survival horror, which also makes it clear how much its systems hold it back (survival horror games do not typically have or need skill points or leveling).
SS1 wins in terms of pure real estate, as Citadel blows away the paltry six decks of the Von Braun (plus Rickenbacker). SS1 is also much more open, as after Medical and Research you pretty much have the run of the station, whereas SS2 is a linear progression. SS1 had a bigger weapon arsenal and they're more fun to use, compared to SS2 which not only has fewer guns but locks you out of most of them through the skill point system. The need to make all potential weapon styles viable has a homogenizing effect that ruins much of the fun of finding a new gun, especially since the later weapon categories of Energy and Exotic trade away versatility for basically nothing. You can beat the whole game with the assault rifle, and not doing so is actually punishing because using the EMP gun and Worm launcher require harder to find ammo and are no more effective (useless against half the enemy types!)
PSI is similarly mishandled as most powers struggle to find a use and are hard to justify the point cost vs. pure damage boosts w/ weapons or accessing more resources via hacking and research. If there's no PSI ability better than a fully maxed out Assault rifle (and there's not) why bother? I always found SS1's straightforward cyberware upgrades more interesting since they reward exploration and don't have stupid tradeoffs.
Graphics wise, early-3D games have held up extremely poorly. The Dark Engine was a poor choice as SS2 doesn't use the engine's strengths well (lighting, basically). The monsters in SS2 are shapeless blobs with mud smeared on them, no skeletal animations or location-based damage (the newest hotness of the day), and mostly just look dumb. The biological levels are also hilariously dated and blocky. Shooting and animation completely sucks, as your guns just jerk when fired and lower off the screen when reloaded; weapons look like they're made from DUPLO. Combat feedback is non existent and fights consist of expending resources to delete health bars. The bestiary is worse as the basic mutants are about the only interesting foe; giant spiders and psychic monkeys are dumb, the exploding droids are infuriating, the flying brains and security robots are a joke. SS1 used digitized sprites which I also dislike, but overall has a better aesthetic. The industrial design of Citidel Station is more visually interesting and unique than the Von Braun, which looks like a straight rip off of a TNG federation ship.
The unlocking panel minigames in SS1 were easy but at least they were something, vs. the one single hacking game repeated endlessly in SS2. I'm not sure how opening a lock and upgrading a shotgun are the same, but according the SS2 they are. SS2 lets you hack turrets and has an economy though, which is nice, and both games have good music (better than this remake, anyway).
