Found an interesting nugget about "Hockey Night in Canada", the long running Saturday night game on the CBC: until 1968, regular season games were always started in progress. Apparently the NHL worried that airing the full game would hurt attendance, not sure why they decided in 1968 to start doing so.
It may have been the NHL, it may have been the two Canadian-based teams at the time (the Montreal Canadiens and the Toronto Maple Leafs), or the sponsor (I think Imperial Oil of Canada, the Canadian division of Esso, was the sponsor at the time). Maybe Imperial Oil only decided to buy two hours of airtime for each regular season game.
I doubt it was the CBC that insisted on this, although until the late 1960's, CBC Radio aired Saturday night NHL games since the 1930's. Perhaps by the time they started being televised, CBC Radio may have been broadcasting the games in full. Maybe CBC wanted to.protect the radio broadcast.
Additionally, when "Hockey Night In Canada" came to TV, the TV coverage began at 9 P.M. EST, but I think the games started at 8:35 EST, so the move of the TV coverage to 8:30 EST probably coincided with the game times being pushed up to 8:05.
But if the Canadiens and Leafs were worried about locally televising regular season games in full, could the CBC-TV network have shown the games in full on most of the network with CBMT-6 (English) and CBFT-2 (French) Montreal, along with CBLT Toronto, running some other half-hour programs and joined hockey in progress at 8:30?
FYI: By the mid 1960's, the four U.S.-based NHL teams at the time were all televising Saturday night away games back to their home markets. As an example, a Saturday night game between the Toronto Maple Leafs and the Chicago Blackhawks in Toronto starting at 8:05 EST (or 7:05 Chicago time) might have been joined in progress on CBC at 8:30 EST, but shown in it's entirety on WGN-9 Chicago starting at 7 Central (8 Eastern).