How? I do remember the big protests in the 90s quite well, certainly, but that was 30-some-odd years ago and when, as we saw earlier, there were 300% more diary farmers. Pressure doesn't last forever, especially as you grow into insignificance.
From what I see the pressure comes from the general public, mostly from having built up this idea that supply management keeps "hormone laden" milk out of the market, combined with the people of Quebec seeing any kind of federal "meddling" as an attack on their people.
When longstanding party favourite Maxime Bernier ran for leader of the CPC, the as you say the nominally free market party, on the platform of eliminating supply management it wasn't just dairy farmers who laughed him out of the room. It was everyone. Same thing happened with the early PPC party, before eventually pandering to the crazies to try and find any kind of support. It was quite clearly non-farmers who wanted no part in seeing an end to supply management.
> because the farmers were fuming at higher freight rail prices against other commodities
Words are only worth so much, granted, but when this occurred it was framed as being necessary for the sake of the entire Canadian economy and to preserve Canada's reputation as a trustworthy supplier. The message sent was abundantly clear that it wasn't being done to appease farmers. How do you translate that into farmers having political sway?