The general population does take PrEP regularly enough to be protected. You seem think that people with substance use disorders are excluded from that, and that belief is grounded in stereotype and bias, not science.
Source: clinical data and professional experience
People with substance abuse disorders are not binarily excluded or included from adherence to taking a daily regimen. However, it is not an unreasonable expectation that someone shooting heroin every week in a flop house is less likely to take a daily medicine than the average population.
The belief that drug addicts can't be expected to take a daily pill is not grounded in stereotype and bias, it's a realistic and down to earth perspective that for some of them, their addiction is crippling their ability to function.
The general population can't be expected to take a pill every day, what would make anyone believe people with SUD are able to?
Source: person who's expected to take a pill every day, and knows other people who are supposed to taste a pill every day.