So im gonna finish part 1 then have to wait months to play part 2? Yea, no thanks, take your time and release it on multiple discs.
Disc space isn't the problem.
If they were to remake it in one single game, it'd either:
A) Take a long ass time to develop, especially at the standard of quality people would expect of it. After the Versus XIII clusterfuck and years of people bitching about that, it makes sense that they'd want to avoid that scenario again.
B) Be half-assed in quite a few aspects, like a lot of content and corners being cut, which wouldn't do the game justice at all, and people would (rightfully) endlessly bitch about that, too.
Doing it in parts is the lesser of two evils and is the most reasonable option: The game gets released in a reasonable timeframe (i.e. not a fucking console generation and a half) and has all the attention to detail and graphical fidelity most people would expect. And they'll inevitably release a complete edition after the fact, anyway.