The problem is multifaceted; first, JS crypto is slow in most current implementations (but will get better with Typed Arrays). Second and more importantly, it is difficult to ensure total security on a page. While loading all assets from a single server using SSL helps quite a bit, there's really nothing stopping a malicious extension from getting in the way and stealing your passwords.

Of course that's true of any website in general so the lesson is: be careful what extensions you install.

Third, browser support is still garbage. Safari has major issues, Firefox is lacking support in critical places, and IE, don't bother.

However the future is bright. With Typed Arrays & Web Workers running on all threads, you can expect 10+MB/s crypto in-browser very soon. That's slow by any other standard but potentially good enough for a browser. Hell, I'm pushing about 3.5MB/s on a MBA on securesha.re, which does 128-bit AES.

Just for disclosure, I wrote securesha.re (and succumbed to a few of these problems while writing it).