First: I like to own my own IP. And I don't want to be vulnerable to a spurious takedown notice if some random griefer decides to take aim at me.
Second: blogger has some hard limits -- official list here: http://www.google.com/support/blogger/bin/answer.py?hl=en... -- and there are anecdotal reports that there is a limit on how many blog entries you can have.
I seriously considered Typepad, but once I worked out how much it would cost it was cheaper to stick with a dedicated colo server (hint: there's more than one blog on the system I run, and there's also a mailman server and some other stuff).
Also: I've been running on a Linux colo since 1996/7, and I really don't feel the need to take a couple of weeks off work to re-jig my entire online presence. This is the first time I've ever had anything like this load level. If it starts happening regularly, I'll reconsider my strategy. Until then, it's a one-off.