A lack of competition. The snag is that Canada has low population density - which means that yes, you can afford to cover most big cities in cell towers, but not outside the city limits -- because you might only serve users who are in a car or on a train as they pass through that cell -- and it's prohibitively expensive to put up a multi-million dollar cell tower to serve users who are passing through for a few minutes at a time.

This is why all of the cell infrastructure is owned by two companies -- because when mobile phone service came to Canada, the fees were high enough, and the costs low enough, that they could afford to build out sites because they were insanely profitable -- in addition to getting funding from the federal government to build out this infrastructure. That's why they're the incumbents -- they have a critical mass of cell sites, and upgrading hardware every decade or two is cheap compared to purchasing/leasing the land and building a tower from scratch (including bringing in power and fibre).