Obtaining the "Monster hunter" advancement requires the player to kill a polar bear, along with many other hostile mobs. But technically, polar bears can only be considered partially hostile, since they're naturally peaceful and only become hostile if attacked (the so-called "passive" mobs). The same happens for wolves, which are not included in the advancement.

As described in this comment in this Reddit post, this is aggravated by the fact that polar bears are a vulnerable population, and as such it is not correct to reward the player for successfully killing them (even if along with other mobs). This can be compared with the removal of the ability to feed parrots with cookies – even if it's just intended to be a joke, and even if it's just inside a videogame, encouraging this behavior in real life must be avoided.

Additionally, from a gameplay perspective it is just counter-intuitive to include such mob in the advancement, and may often require players to look up the required mobs instead of figuring them out on their own. None of the other required mobs are real-life creatures; all of them are fictional to some extent – some, such as zombies or skeletons, can be seen in other fictional universes; some are Minecraft-specific, such as blazes or ghasts; but none of them are real creatures. As such, having a real-world animal be part of the advancement doesn't fit its pattern (at least in it's current arrangement, which may change in the future).