I think it's too simplistic to just say that Wonderland has no competition and that's why attendance is so strong. True enough that there is no other major theme park competition, but if you speak to their marketing and sales folks, they will list off at least ten major competitors in the GTA (from the zoo - one of the largest in North America - to Ontario Place, etc.).
Remember that family attendance and group sales is a major priority, and Wonderland does very well on both counts. They have a massive assortment of kids rides, two kids waterpark areas, solid shows, etc. They have kept pricing very affordable for both season passholders and day guests (with Web pricing and discounting). The new food options for this year only boost the park's family appeal (can you see a bunch of local Vaughan teens opting for the buffet over cheap pizza?).
For a family like ours to spend a day, or half day, out, Wonderland is a great choice. We went nearly a dozen times last year, and rarely did we hit any coasters - mostly the same routine of Nick Jr. rides, Kidsville, Dora show, etc. And you can tell that there are lots of others like us. In fact, I think one of Wonderland's main challenges going forward is adding new attractions that entice people to come, but not ones that will only draw more teens and local kids. Cedar Point has a comparatively small local market, so there really aren't a lot of local teens that will overwhelm the park's thrill rides like Wonderland.
I am sure there are other parks in markets similar to Toronto (SF Great America, for example) that have a similar situation in terms of competition (or lack thereof) but don't draw the same numbers. Face it, like it or hate it, Wonderland does what they do very well, and the numbers back that up.