PK Wrigley was as ignorant about the cost benefits of television and sports as almost every other owner of the time was. He basically gave the rights away for free to WGN (and earlier, had Cubs games on both WGN and WBKB for nothing). He WAS very insightful about TV being free advertising for the team, but he made no money off the TV rights.
@Dick Allen is absolutely right. The teams were basically on equal footing until the early 80s, when the Sox moved off free TV. But even then, the Sox were still right there with the Cubs "83 and '84 -- they were the first team in Chicago to draw over two million fans two years in a row in those years.
But the combination of the Tribune Company's national reach as a superstation, along with Harry Caray moving from the Sox to the Cubs began the real momentum of the North Side as the dominant team. The Sox came back with the move to the new stadium in '91. But after the '94 strike, where JR was the most prominent union-buster on ownership side, the Sox were diminished even more. And the Sammy Sosa phenomenon in the late 90s, culminating with the '98 season, really killed the Sox.