I think it was just that the QMJHL was considered far inferior to the other junior leagues at the time. It should be noted that Larouche's two linemates that year, Jacques Cossette and Michel Deziel, both also cleared 200 points that season but never were able to stick on a NHL roster.
The May 17/74 issue of the Hockey News had a article previewing the draft, and the two prospects it most hyped were Ron Chipperfield from the West and Bill Lochead from Ontario. Due in large part to all the underage prospects being taken, those two really slid in the draft, Lochead going at #9 and Chipperfield at #17.
They also had a list of the top 100 prospects, though I'm not sure if it's an official scouting list or just one writers opinion. The first three are Chipperfield, Lochead, and Bruce Affleck, who was playing in the NCAA, and was drafted at #21. Top prospect from the Q from Richard Nantais, was rated 9th but drafted #24. Amazingly, Greg Joly is only rated #7, but this was before Regina won the Memorial Cup, which would have raised his stock.
Larouche is not listed because he was underage. There is a short list of the top six underage prospects at the bottom, but Larouche is not on it.
Under normal circumstances, Mark Howe and Dennis Sobchuk would have been the first two picks, but the WHA already had them. The top underage prospect was considered to be Pat Price, and apparently Washington wanted him as their first pick. But once again the WHA beat the NHL to the punch and signed Price to a contract rumoured to be more money than any NHL defenceman was making.
This prompted the NHL to hold their draft in secret by telephone, with Clarence Campbell calling each team when it was their turn to pick, and reading off all the players who had been selected since their last pick. This accomplished nothing, as word the draft choices leaked out anyway, but it did tick off Punch Imlach enough for him to pull the Taro Tsujimoto prank.