Because it was a lot of very clever mathematical trickery. ED is using a lot less in the way of smoke and mirrors and doing a lot more of the actual work Frontier very successfully pretended it was doing.
Well, exactly, how hard can it be to put similar, procedural-only stuff into single player?
Elite II didn't need other human players to make it enjoyable. I don't think many people would complain if the Multiplayer galaxy was a bit different from the single-player galaxy, they would and probably should be different 'universes' anyway.
So really, even though the online game is doing lots of cool dynamic non-trickery, there is no reason why the offline single-player can't rely on procedural 'smoke and mirrors' to make up for it - just as all previous offline, single-player Elite games have.
The only reason I can immediately think of for trying to keep both offline and online game-modes in sync is so that experience and assets gained in one mode can be moved to the other [ e.g. level up offline and then jump into MP ].
The thing is I never wanted or expected to toggle my character, money and ships between offline and online play, exactly because it brings up this whole issue of an offline 'fixed' world verses a dynamic online world.
I hope a reasonable solution/comprimise crops up : a totally offline single player or a periodic non-essential manual synching of single-player , if you want.
I want to be able to fly off into the galaxy and explore in my own game without expecting every, single, interesting thing to already have been found, scanned, mined dry and built on by half a dozen other players by the time I get there.