I can think of a few good reasons to stay in Canada:
1) Tuition -- Tuition in Canada ranges from 7000 - 22000 (with most schools around the 10,000 range). Why pay 45,000/year when you don't have to?
2) Admission standards -- due to Canada's small population, less people apply to law schools and thus there is less distinction between the best schools and the worst schools in terms of admission standards. A 166 could get you into any school in Canada
3) Rankings -- there is less pressure to get into a "good" school in Canada. Though a school's rank is somewhat important in Canada, it is no where near as competitive as it is in the US
However, I've heard that significant scholarships are more common in the US, so if you're getting a full or partial ride, that may affect your decision to study in the states
Lower Admission standards, less emphasis on rankings, less pressure, LESS GAP BETWEEN THE RICH ("good" school) and POOR ("bad" school), Such typical Canadian socialist thinking. You're certainly entitled to it. That said, my 3000/year tuition did lead me to stay in Canada.