My guesses (and these are merely guesses) are that it was unnecessary to the story and it would have cost more to recreate still another staircase set. (Might have made things more confusing to the audience, as well.)
Again, just guesses (and not thoroughly educated guesses, either), so make of that what you will. I'd be happy to read others' ideas.
>>In the movie only appears the first grandstaircase, but the other not, why?<<
What for? As Doni suggested, it wasn't a part of the story arc so there was really no reason to give it a lot of attention. As expensive as the movie was to make, it's a wonder they managed to show what they did without the studio pulling the plug on the whole project.