Why does the Tycho-2 catalog not include Hipparcos stars? Both use the same data. But at least, 90k stars are lost in Tycho-2. 
Why?
Could you give a source or example: The Tycho-II catalogue paper by Hog et al. (2000) suggests there are about 19,000 stars in Hipparcos or Tycho-I that are not in Tycho-II. This can be because they are too bright (but that is not very many stars) or for other reasons that are not clearly specified (binarity is an issue I think).

Looking at this website, it suggests 4% of stars did not have proper motions and could not be assigned epoch 2000 positions in Tycho II but I don't think these are missing.