Because (as someone put it to me recently) Most republicans are at work when the pollsters are calling during the weekdays and on the weekends they are out w/ their families - that’s not to be taken the wrong way since dems work too but ya know...
Plus, the polls are done on landlines, not cell phones.
So whatcha basically have is a bunch of pollsters calling people in the daytime on landline phones who are hanging out watching Oprah or “The Young and the Restless.”
Dandy sample there!