On the one hand, one can attribute this to the wildly growing popularity of blogs.
On the other, it looks like a classic case of everyone knowing that backups are essential, but actual practice lagging a big.
And one can note, as David Berlind does, that if blogs were kept in a standard open format it would be easy to move among blogging platforms.
Me, I have a new policy of exporting a backup of my entire blog every time I post something.
-jsq