Some say the move was purely technical - Google is leaving its angled bracket XML partner to move in with the younger, sexier JSON. Others say the move was a business decision - Google can't afford to let advertising opportunities slip by.
I think Google did solve a technical problem. The problem was unregulated XML data. The solution was an AJAX control. Control is the essential word. The scripts that Google requires for the new search API keep them in control of the search results, the branding, and the advertisements. Producing data from an XML web service doesn't afford this level of control, but producing behavior through a ubiquitous scripting language does.
This could be the first of many backlashes against the bohemian goal of creating free-for-all mashups. Companies with balance sheets and stockholders aren't looking for a technology that will give them data interoperability, but a technology that will protect their investments.
AJAX is the DRM technology for the Web 2.0.