For example take the example of Chicago and then compare it to Toronto...
Chicago has a more solid identity with easily quantifiable things to do. People know Michigan Ave, John Hancock Center, Willis Tower, Millennium Park, the EL, the Chicago Art Institute, the Northside neighbourhoods, Adler Planetarium, Lollapalooza, etc.
Toronto in comparison, most can come up with the CN Tower, maybe Yonge St, which has a third world market feel south of Bloor, perhaps the Toronto Zoo or the Hockey Hall of Fame. Other travel or tourist destinations are rarely publicized by outside media, places like, Kensington market, the Beaches, the Queen West galleries, even Dundas Square or the numerous big festivals in the city are relatively unknown both outside of Canada and definitely outside of the country..
In some ways Toronto doesn't promote some of its best assets well. Part of it is that it's harder for a city like Toronto to become "iconic" like New York City or London can, due to a lack of assistance from the American Entertainment and Publicity juggernaut. In addition the city is in a rather dynamic period in its history right now, so things are still developing at a very rapid rate and we don't know what the picture will end up looking like, which is why the city's image is slightly ambiguous to most right now. And that in itself makes the place hard to communicate to outsiders as a premier travel destination.
Right now, Toronto may have more inbound traffic, but a lot of that is either visiting relatives in the city (Toronto is still considered a place to live not visit), for business, or to connect elsewhere in the country (especially true for international flights). That isn't to say there isn't a tourism industry in Toronto. There very clearly is and it is growing exponentially year by year. More and more people are choosing to make Toronto apart of their travel plans, or make the city their travel plans, full stop. It's only going to continue getting better for Toronto from here on out, but for now, Chicago still has an edge as a destination.