Why is Unnerving Prescence a Trapper perk?

Because a lot of perks are based on the canon for the characters, not just how they work ingame.

Trapper is often depicted as being a tall, looming, but calculated figure who kills Survivors by planning ahead and with their brute strength. In a similar light, "Brutal Strength" comes from hit raw strength, and "Agitation" comes from brutality and desire to hook Survivors; in a sense, "Unnerving Presence" fits more into his character since their Terror Radius often means they are trapping or looming nearby.

It is also important to consider how older DBD was.

While their traps were brighter and took longer to set, maps were generally darker and addons could make your traps blend in very easily. Grass was also naturally higher, which made hiding traps much easier. A lot of Survivors also did not know how to loop, and good information perks did not really exist, so stealth was considered the main way to play the game. For those who did know how to loop, Trapper could shut down infinite loops whereas no other Killer could. Trapper was definitely not an insanely strong Killer, but they had a decent place in the game, especially compared to the old Wraith.

Outside of all of this. Unnerving Presence fits because Trapper is the poster-child for DBD. They are used on all of the advertising and are slapped everywhere, they have a presence that no other Killer really has in that regard.