I also thought about "nostalgia" and that some great characters will most likely never come back...But the more I'm thinking about it, the more I understand, why Crystal had to change everything.
If one of the older characters, let's say Zip was in the new game, people would most likely compare this game to the Crystal-trilogy, or would try to find some parallels - and I think that's exactly what Crystal didn't want for their new game.
By changing every single character and the story they are somehow creating a brand-new game with a well-known heroine, that still feels a lot more like a real TR than the whole trilogy (to me, at least). The most important aspects were kept [such as: puzzles, platforming, raiding tombs (even if the reason, WHY she's raiding tombs is a lot different this time)] but others were binned.
Imo this game somehow feels fresh and nostalgic at the same time and thus both the old fans will be pleased and new ones will be attained (not sure if that's the right word in this context)