I doubt the plowing is a big factor since we actually plow our bike lanes much less frequently than other cold countries like Finland and they paint their bike roads. Then again, you have a good point about paint being invisible because it gets covered in snow.

In Finland they use lamps above bike lanes to add colour and signage over the snow. I just think the gray asphalt makes it feel like the area is still for cars. That psychological effect is a huge part of good street design.