Government pricing will keep the black market in place (the black market has very little overhead so they can compete easily with licenced growers). 

Quality/strength of the product will keep the black market in place (who wants that mediocre government-approved oregano?). 

The black market infrastructure for distribution and 'retail' is already in place. We already know and love our dealers. Who wants to deal with all the restrictions of government-approved retailers? Older Canadians will remember how liquor stores used to operate (definitely a repressive environment and a hostile retail experience).

But above all, those under 18/19 will keep the black market in place (everything remains status quo for teenagers). 

Marijuana is a de facto unregulated product now and 'legalizing' it will only make it less likely to be controlled. How will all these silly regulations be policed if we can't police illegal drugs  now? Why should we bother?