I think part of the reason is because Can Am comes out with new models that have more power and other feature improvements every few years, so owners are always upgrading to new models. There is no reason to buy a new Honda or a new Yamaha, because they never change. The other brands, but to a lesser extent, are the same way. Polaris and Can Am are leading the charge for new improved ATV models.
I've owned nine Can Am or Bombardier ATVs. Three of them were DS 650s (2001, 2003, 2005) that I desert raced every weekend. Some of them had an estimated 7000 miles on them when they were sold, and they were absolutely bulletproof. Ride them as hard as you possibly could, roll them numerous times, and they still keep winning trophys, and finish every single race. One was a DS 450 that I bought used. That ATV was also bulletproof. My son missed a turn at race speed and uprooted a 3" diameter cedar tree and drug it 40 feet. He took out the radiator and both a-arms on the left side, but the rest of the ATV was fine, including that high tech aluminum frame. The other five Can Am ATVs have been 4x4s (three Renegades and two Outlanders). They also have been mechanically bulletproof. I have never had a mechanical problem with a Can Am 4x4, including the frames.
The problem with Can Am in my opinion is that they can't figure out electronics to save their life. Four of our five Can Am 4x4 ATVs have had power steering, and all four of them have been back to the shop for power steering issues. Sometimes you get the idea that it is rare to take a Can Am for a ride and NOT have a warning light come on. Can Am over engineers their ATVs, with too many electronic features, too many warning lights, and too many electronic nannys. Other than that, I think they're as good as any ATV in the industry for reliability.