Interesting report, although I didn't realize that, for the purposes of this report, Seoul was in Europe... (p.12)
Candidly, I'm a bit surprised by Vancouver's ranking, but then again it is one of two designated international finance centers in Canada with incentives designed to attract international finance activity. Also as Yume mentioned, private banking is big. But remember that part of the results of this report are a survey sent out to a sample of 1690 financial professionals, which seems rather a small sample size...
In general, though, I would guess that ratings of US cities tend to suffer because of regulation and disclosure issues (eg Sarbanes-Oxley). I was exploring listing a company on the AIM board of the London Stock Exchange a few years ago when Sarbanes-Oxley was enacted in the US, and it was causing many foreign firms to consider moving their IPOs from the US to London.
Also, this survey measures global financial centers, and I would think most US financial centers are more focused on serving the domestic market. This would also explain Tokyo's ranking, which in absolute size is bigger than Hong Kong as a financial center, but ranks behind Hong Kong on this index since it primarily services the Japanese market.