Couple reasons:

1) sIFR, Cufon, image-replacement, etc. each have their own problems. Hard to setup, can't select the text, tedious to maintain, requires opening Flash CS4, etc.

2) @font-face is the future, but most paid font foundries refuse to support it b/c it requires the font file to be linked to on the server. So designers can legally use free fonts with @font-face but not fonts they've purchsed.

So, Typekit is an attempt to keep the font foundries happy by hosting the font file and preventing unauthorized downloading, while keeping an eye on ease-of-use and the future of the web by doing most of the @font-face heavy-lifting.