Honestly...probably because there's not a lot of demand for optometry schools here. Waterloo allows 90 students a year, and last year they had something like 300 applicants. That's a pretty standard acceptance-to-applicant ratio for a professional school - it's not like there are thousands of people trying to apply to the one school. (Actually, that's even a pretty high acceptance rate - 30%, when med school is more like 10%). I'm not sure what Montreal's is, but considering it's French, it doesn't really apply to this topic.
Now, to be fair, the applicant number might be lower because so many people think, "Well, since there's only one school it must be hard to get into!" So they just apply to American schools. Who knows.
I'd really like it if they opened a school on the west side of the country, like at UBC or something, but it probably just doesn't look like that many Canadian students want to be optometrists, and it doesn't make sense to open a second school if it won't be cost-effective - or if it means lowering acceptance standards because you can now have 180 students out of 300 of the same applicants.
Also, we have approximately 11% of the United States population, and 10% the amount of schools (if you include Montreal, otherwise it's 5%). So as far as school-to-population ratio goes, they're not far apart.