It's because Quebec has always been a trading and free-trading nation. The province was at the forefront when the matter was debated in the 1980s. We can't be against that, of course. Any opposition would be in the event that Quebec's specific identity would disappear. That's where the problem lies.
I'll move on. There have been surveys of businesses, including HEC Montréal, which doesn't oppose trade, in life.
Ultimately, it was said that the main obstacle wasn't the issue of regulations; it wasn't really that, but the issue of distance.
Have you looked into this matter?