Why is this a UK 15?

Rating: 8/10 (26 October 2019)

2 hours of mainly sitting in the cinema at an angle, that is how awkward and disturbing this superbly acted and directed movie was.
But why was such a potentially manipulative and violent film only awarded a 15 rating when Alien, that whilst terrifying, was not disturbing or liable to influence vulnerable young minds given an 18?