Sundar makes ~200M as the CEO of Google whereas DeepMind sold for ~400-650M. There's plenty of monetary incentive to take the job, not to mention more power to set the company direction through resource allocation. And it's clear that there's been a PR campaign being set up to push out Sundar. Maybe Hassabis is a contender because he's been getting some pretty serious press since the beginning of the year (which is when the Sundar article grumblings started).