You asked why: there are basically two categories of Canadians who do law school in the states:
1) Top-tier - Some students may go to T14 schools thinking it will help there prospects in Canada. This isn't necessarily the case; they still have to go through the NCA process, but many of them do so nonetheless.
2) Back door - Admissions standards are significantly lower in the US than they are in Canada. This isn't to say that the US doesn't have plenty of fine and elite law schools that are hard to get into, but Canada simply does not have anything like 4th-tier schools where just about anyone with an undergrad degree can go. Even the lowest ranked Canadian law schools require about a 160 LSAT and a minimum of 3.4 or so GPA. Plenty of Canadians who can't get into our schools will go down south for a back door into the Canadian law scene. This usually backfires because it's difficult to practice in Canada with a US degree (you need to go through the NCA process which requires extra courses and exams, and is time consuming and expensive) and because it's really frowned upon in the Canadian legal community, so unless your Daddy is a partner on Bay Street, you're pretty much out of luck.
Also worth noting, Canadian law schools cost $10k-15k per year (except Osgoode and U of T, the most elite schools, who charge around $20k per year) so Canadians who go south pay for it big time.