I've found a 2014 article by the BBC that explains why fish are not expected to survive beyond about 8,000-8,500m:
> It rests on the particular molecular mechanism they use in their tissues to withstand crushing pressures. To go deeper would require fish to evolve some other mechanism, the team tells the journal PNAS. The all-important molecule is a so-called osmolyte called trimethylamine N-oxide (TMAO). It is what gives fish their “fishy smell”. TMAO acts to stabilize the proteins fish use to build and maintain their cells. Without its presence, the proteins would be distorted by the high pressures found at depth and stop functioning.
Other organisms like crustaceans are able to occupy deeper parts but they use 5 different osmolytes for this purpose whereas fish have only ever been observed using this one.