When 0 HP is not the same as dead
One of the biggest advantages to Power Word: Kill is that the target is dead, not just at 0 HP.
I can think of several situations where an enemy having 0 HP is not the same as the enemy being dead.
(Side note: This is in large part why it could be argued that this spell is not intended for PCs, as PCs don't die at 0 HP. However, the question is asking from a player's perspective, so this answer focuses on that aspect.)
While it depends on your DM, not all enemies die when they reach 0 HP. Optionally, the DM may choose to have death saving throws for an enemy; or worse, there may be an enemy healer in the battle too.
Think of all the times you've seen someone cast Healing Word, or put a Goodberry or Healing Potion into an unconscious ally's mouth. Such a tactic is very powerful at getting another PC up and running immediately.
A DM is free to use that same strategy by including powerful healers as part of an encounter. Knock the big enemy down to 0 HP? Good job, until it's the healer's turn and brings them back up to positive again.
Vampires have the Misty Escape trait:
Misty Escape. When it drops to 0 hit points outside its resting place,
the vampire transforms into a cloud of mist (as in the Shapechanger
trait) instead of falling unconscious, provided that it isn't in
sunlight or running water. If it can't transform, it is destroyed.
When fighting a vampire, Power Word: Kill can be the difference between a challenging encounter and a recurring villain.
Theik's answer covers this well.
In short, a Polymorphed or Wild Shaped creature reverts to their original form when they reach 0 HP. With Power Word: Kill, they just straight up die (and their dead body reverts).
It is irresistible
Many spells have the ability to save to reduce or avoid damage. Some creatures have Legendary Resistances, which allows it to save a limited number of times per day, even if they fail their saving throw.
No such save exists for Power Word: Kill. The only condition is that they have 100 HP or fewer, and then they die.
It has only a Verbal component
There are no material or somatic components to this spell, which means you can do it even if you are missing your components pouch or have your hands occupied with other things.
Not all tables pay attention to such restrictions, but if your DM enforces having a free hand (or something that fulfills the requirement) in order to use somatic and/or material components, this makes this spell potentially very useful.
Situational, but simple
In short, the spell is not a generic "I win" button, but rather a situational tool that can be used in a variety of circumstances.
It is not going to be the best choice all the time, but it can easily be the best choice some of the time.