It might be due to creatures with the undead trait die immediately at 0 HP (not to dying 1 condition). So they might have wanted a similar vibe to actual creature revenants but without removing player character survivability. Also the actual undead heritage you are referring to gets Basic Undead Benefits which notably stops them from dying at 0hp immediately, but also gives immunity to death effects and better vision, undead hunger etc. On the other end, an Undead heritage cannot have stabilize cast on them but a revenant can, undead are destroyed if they die so cannot be brought back to life again, whereas the Revenant can keep being brought back to life and become Revenant^2^ I guess.
So the Revenant background gives some aspects of the undead without giving all the full blown benefits of an entire heritage option and not as many drawback either. I'm seeing it now as Revenant == Undead-lite.