There's several reasons for this. There are many things in the blood stream that would be toxic to neurons if they passed the blood-brain barrier. Also, blood spilling out of a hemorrhagic stroke doesn't oxygenate the brain in the normal way though capillaries, where the blood ells progress to veins and then eventually are re-oxygenated. Blood spilling out onto the brain causes most neurons downstream to be deprived of oxygen and nutrients.