I disagree with the idea of this game's launch on Steam being its big time in the limelight. IMO, this game has simply been in development for too long, had way too much attention put towards it, and had way too much polish put in it for the majority of people that would play this game to think of it as an exciting new release. At this point, this game's launch on Steam would be much more like Cosmoteer's or Dwarf Fortress's launches, where many people just went "awesome, this game is on Steam now." TotalBiscuit's video on this game a decade ago already sort of broke it out of the realm of pure obscurity, but SsethTzeentach's video 3 years ago sealed it and made this game a well-known title among games with similar audiences like RimWorld, Kenshi, Dwarf Fortress, or Factorio. Practically, this game released years ago and people perceive new major updates more like Terraria's free major content updates than the game taking forever to finish. Alex and the other developers have simply built a game that, after years of polish and development, doesn't need Steam to have a degree of success--only the developers can say whether or not this game is successful--and have an audience. Releasing this game on Steam would just be a nicety at this point.
So in terms of answering "Why isn't the game on Steam yet" - What exactly are you saying, in regards to the OP question?
In layman's terms and (in your opinion) why hasn't Alex released via Steam?
My suggestion was simply him just wanting it to be finished before the wider steam community got their hands on it. I'd suggest that's a fairly reasonable assumption, but only Alex can confirm either way.
Ah, that's a fair point; my bad.
My assumption as to why this game isn't on Steam yet would be that it's simply not done yet, Early Access has a horrible reputation, and people would
absolutely complain about this game taking way too long to get updates. Starsector itself has been in development for longer than Early Access has existed, and the game regularly takes more than a year to get updates. While I do agree that the website and the way you buy this game are a bit... old-fashioned, they're a natural filter that prevents anyone from not knowing what they're getting into here.
Also, I definitely see where you're coming from; I just feel like while the game would certainly have a lot of new eyes on it, it wouldn't really be a gigantic influx of players since the game itself is still fairly niche. The portals people would have to find this game
anyway(YouTubers, word of mouth) already exist, and it would be best for people that--as the OP pointed out--are intimidated or weirded out by how you buy this game currently.