They like using the service Google provides and they really want to continue paying for it. Not everyone gets their rocks off serving a popular website and tuning the dozens of arcane knobs to make it hum as well as Google can.

They probably don't want to pay $$$$ for someone else to move it and/or host it. Migrating years of posts and all the comments... that's a decent sized job. Even if the export/import to some other blogging engine was nearly perfect (Hint: it's not all roses and perfume), there's a dozen or more billable hours involved to make it run exactly the way it did before on another host. That's probably more than he spends in a year at the current location.

...and so on. There are plenty of good reasons not to move it.