Maybe I'm misremembering things, but back in 2004/2005 Smogon was pretty much THE forum for competitive 3rd gen, and Netbattle was THE way to play Pokemon online (previously people had used IRC bots).
Back then, the community for online battling wasn't nearly as big (perhaps in large part because the GBA had no online function, hence the requirement of using a simulator, and the internet as a whole had discoverability issues back then), so I guess you could say that Smogon was a big fish in a small pond, but if you were one of the few hundreds of people that played competitive Pokemon online, the discussion basically all took place on Smogon. There were some other Pokemon forums that had some competitive discussion purely by virtue of being big Pokemon forums to begin with (like Gamefaqs, Pojo, and Serebii), but those forums tended to have more discussion of in-game stuff and most people's version of "competitive" play just meant having a decent team for link cable battles (not breeding for good IVs or even having optimized EV spreads), so Smogon (as a forum that was dedicated purely to competitive Pokemon discussion) drew a lot of people from these forums that wanted a place to actually discuss competitive Pokemon. There was also the TVsIan forum where people discussed competitive battling just because it was the "official" Netbattle forum, but as I recall that community was kind of decaying and Smogon pretty quickly sucked up most of the people from there.
So like most people, I joined Smogon because it seemed to be the only place on the internet that was geared primarily toward people who wanted to discuss Pokemon as it was played competitively on simulators. That's what got me in the door, anyway.
There's also a cultural component that made Smogon a bit of a draw. For one thing, it was pretty elitist, which I liked at the time: I was tired of having discussions on general Pokemon forums where randoms would often interject to say that playing the game competitively meant we were playing it wrong ("obsessing over the details of EV spreads isn't fun, why don't you guys just have fun, it's a video game, why do you care so much about math, why take it so seriously), and Smogon's elitist attitude was sort of the perfect counter-weight to that (play to win, get good or get out).
That brand of elitism was just part of a culture that I recall as being very sophomoric. Early in the days of Smogon, when the site was still finding its identity, you had some teenage wannabe philosophers trying to have philosophical discussions on "serious topics" so that they could sound impressive and smart. In another category of discussion, I recall there also being quite a bit of personal storytelling, sometimes this took the form of thinly-veiled brags in the form of stories about how they got so drunk and had these crazy party experiences, or telling stories about their sexual escapades, but a lot of it was just telling stories about weird and crazy things that happened to them. I recall one of the more amusing threads in early Smogon history being from a guy (who, as I recall, didn't even really play Pokemon) talking about his experiences in India, and finding the most colorful metaphors to describe the various disgusting things he experienced there. There was also a lot of crude and mean humor, like people finding creative ways to insult each other. Though much of it seems very sophomoric looking back on it now, at the time it was kind of the perfect blend of ingredients to make a 14-year-old say wow, all these guys are so cool!
By the way, I hope the above paragraph doesn't sound like I'm trying to dunk on anyone; for all of the faux-sophistication of some users who tried way too hard to seem erudite, I did encounter some really intellectually stimulating material (if I recall correctly, it was through Smogon that I first discovered Paul Graham's essays), and the threads that were just people posting stories from their lives did make for a lot of interesting reading in an era before blogging and social media was really a thing. In a way, reading stories from various internet strangers was a basic exercise in empathy; it gave me the chance to see the world through other people's eyes in a time when I didn't spend a ton of time offline interacting with people who had different perspectives.
It could also very well be that I'm misremembering how big a deal all of that was in relation to what the forums as a whole were, but in an era when I was 14 years old and basically every major Pokemon forum was trying to be family-friendly, Smogon's edginess was an undeniable draw. Smogon was where all the "cool kids" were.