This is the anatomy of a hair follicle
You can see that there is no muscle to control the size of the pore where the hair exits the skin, so there is no way that the body can control normally the size.
The primary problem is not with the Propionibacterium acnes but with excessive oil production which then causes a blockage in conjunction with dead skin creating a fertile environment for the bacteria to proliferate.
Commensal bacteria on the skin are not controlled by the immune system ( white cells, antibodies, complement etc can't migrate through intact skin to reach the bacteria ) but by competition with other bacteria, moisture etc.
Oil or sebum production is driven by sex hormones testosterone, 5-testosterone (DHT) and 5-androstene-317diol, and unless there's some evolutionary advantage to do so, there won't be any selection for those individuals who less likely to produce excessive oil. Since male hormones are important, then evolution may be selecting more for those with high male hormone levels which produces more of an advantage then the temporary disadvantages of acne.