Well for starters, Reincarnation was changed in WoW to require that an Ankh be held by the Shaman before death, so maybe Cairne did not have one as that would have been a magical advantage, against the rules of Mak'gora.
Vol'jin's pact with Bwonsamdi could have prevented resurrection from being an option to him.
As for Liam, the only thing I can think of is this: We don't fully understand the process of resurrection. All we know from the game text is that it "returns a soul to its owner's dead body." This is done through all sorts of magical types. Perhaps the manner in which Sylvanas slayed Liam prevented his resurrection by typical means?
C'mon