Because they just make the calls and that was the distribution for the period. They then re weight to get what they believe would be the actual turnout. If they kept hanging up on republicans until they got the right amount of dems (or visa Vera’s if it turned out the other way) , it would take forever to complete the poll.
They assume that it will be a lower turnout of Republicans. Assumption is life’s lowest form of knowledge. Enthusiasm factor favors Trump. This poll is turning into propaganda.