My understanding from conversations I've seen about Google Reader is that the problem with Google is that every few years they have a new wave of infrastructure, which necessitates upgrading a bunch of things about all of their products.
I guess that might be things like some new version of BigTable or whatever coming along, so you need to migrate everything from the previous versions.
If a product has an active team maintaining it they can handle the upgrade. If a product has no team assigned there's nobody to do that work.