Common liberals have not been bright enough to figure this out.
Liberals may be misguided but in my experience the "common liberal" is brighter than the "common conservative". Whereas the "common liberal" bases their beliefs, right or wrong, primarily on their desires for bettering society, the "common" Bubba conservative beliefs are shaped by prejudices and fears.
BTW, the poster you replied to is about as liberal as Kevin Trudeau is an actual health authority.