Let me take you back to 1987. Predator, Robocop and Beverly Hills cop 2 were just 3 of the films you weren't allowed to see cos you're 11. You've started high school. La Bamba and Risk Astley are the only songs you ever hear. Your mum still buys your clothes. You are not cool. You meet a bunch of other guys who are not cool. You meet to have lunch in a history room to avoid the mentalists that fill your home town. Somebody introduces you to Fighting Fantasy books. From there it's a short hop to dungeons and dragons and Runequest. Then someone points out you can buy figures. But you have to paint then! But they show you how in these magazines! Look those guys have guns! Those are cool! If we buy them we'll be cool! And there is game to play them with! Let's do it!
Well, that's one theory. I'm still trying to be cool. Still hasn't worked.
2. It's a better game.
All 5 editions since Rogue Trader are essentially the same game with different sections altered or tacked on. It's like having a girlfriend who gets her hair done every week and buys a new dress but you know she never changes her underwear. It looks new and sexy but you know what your going to find underneath.
Rogue trader is clunky and clumsy and can take ages to play. The vehicle rules are terrible and they had to change them twice! So that can't be the answer.
3. The fluff was better.
Better how? It certainly wasn't very clear. The modern fluff is very rigid and the story is very tightly controlled. In RT it was kind of all over the place. You had a graphic story about Logan's world. Bit of info about the Imperium of man, some major alien races and some mad flora and fauna but there wasn't an all encompassing scheme. It seems like a load of mad ideas thrown at a wall and most if them stuck. It certainly left a lot to your imagination (and this is a very good thing!) but I'm not sure better is the right word.
One thing that occured to me is 'the scenario'. In today's 40k the scenario is a very basic little frame to allow you to alter your games slightly. Roll a dice, fine you come on from a corner. It's basic, everybody knows what's going to happen. Most players have already worked out which units in their army will be holding which piece of ground for how long in order to score the victory points to win in scenario 4 before they even painted a model.
I think the real reason may be because see it as such a massive missed opportunity.
There was a time where 40k didn't know what it wanted to be. It wanted to be a table top role-playing game but also wanted to be sci-fi wargame. And for a while it could be both those things. But not for long. Because we'd bought more figures and more tanks and we wanted more stuff and GW gave us what we wanted and led to the world dominating table top monster we have today.
After all, what is a Rogue Trader? A person who shrugs off perceived imperial wisdom and goes off hunting for endless possibilities at the edge of the known galaxy. Doing things his own way with whatever forces he feels fit and making it up as he goes along.
No wonder we don't call it 1st edition.