It's also impossible to overstate how impressive the graphics seemed back when the game was released. It still holds up, and modded ME1 manages to look better than a lot of much more recent games. (e.g. Solasta, 2020. lmao.) ME1's art direction and design language was really on-point, though sadly ME2 and ME3 took it in a more conventional direction.
This reminded me of a fairly important point when it comes to the popularity of ME. At the time, ME1 was something entirely new. You didn't have the knowledge that the sequels would fuck up and that the ending would be garbage. It was a new universe with cool aliens being rendered in what seemed like photo realistic graphics at the time. The world building was very impressive and the writing was generally of high quality (not flawless, but good overall). Everything in the codex was something that you could hope to see for yourself in the sequels. Because we did know there would be sequels, since it was planned as a trilogy from the start. Shortcomings could be excused because Bioware had made something no one else had at the time, and with sequels confirmed it was obvious (to people at the time) that they would use what they had learned from the first game to improve. This was before "AAA sequel" almost inherently meant decline. Mediocre gunplay? Easily fixed in the sequel, they already spent all the time on creating the universe and laying the other foundations for the series. Empty planets and bouncy Mako? They will have much more time to create unique worlds since it's just a matter of creating more assets. The engine and all that is already done, after all. Dialogue wheel? A popular suggestion at the time was that they should let you press a key to display the actual line of dialogue instead. It seemed like a no-brainer. Choices and consequences? It was proudly marketed that they would be a core part of the trilogy.
You can't get this now. Bioware had pretty much created the Star Wars of the current generation. People were actually optimistic to see more of it. We didn't have the sequels that were practically engineered to piss off the old fans. Most people expected Bioware to keep the good parts and improve what was lacking.
The world building and the codex were what hooked me, personally. It was a promise of something we would see in the future. We have the humanoids working already, so now they will surely give us more unique party members like the hanar, elcor or volus and let us find out more about them. And I don't mind pure text or sprites, but seeing these species as high quality life-like 3D models was new. They weren't just enemies for you to kill, either. High budget space opera RPGs weren't exactly common at the time. Maybe that sounds like graphics whoring, but presentation does matter. This was before graphics had plateaued. Some of us still had hope that the future would allow graphics and complexity to coexist.
Was all this fucking stupid in hindsight? Clearly. But back then, I was optimistic that Bioware would use the foundation to make the sequels better and more complex RPGs. I wasn't the only one with that mindset. This is something that needs to be kept in mind when you wonder why that partly brilliant but in many ways mediocre ME1 got the fans it did. It was new, different and a promise of improvement.