Why is the outer membrane of prokaryotes more permeable than plasma membrane and permits the passage of small molecules?

As a phospholipid bilayer the lipid portion of the outer membrane is largely impermeable to all charged molecules. However channels called porins are present in the outer membrane that allow for passive transport of many ions, sugars and amino acids across the outer membrane. Three porin molecules cluster together and span the outer membrane to form a narrow channel through which molecules smaller than about 600 to 700 daltons can pass.