Some might argue that it doesn't. Every winter, Ottawa's Rideau Canal freezes to form the world's largest skating rink. Just about every city park sports an outdoor ice surface. But almost all of the city's ice is natural. On warm days, the rinks become puddles, as shown here.
This year, Ottawa is unlikely to have skate-able outdoor ice until January. In Toronto, the outdoor rinks have been open since the end of November.
It might seem odd that Toronto, which is three to five degrees (Celsius) warmer than Ottawa, has an outdoor skating season that lasts a month or two longer. Yet the very fact that Toronto is warmer than Ottawa may be the reason why it has better outdoor ice.