Google makes no money if they can't show ads to people, and at the time Comcast and friends controlled Google's access to a majority of US audiences that Google's customers (advertisers) wanted their ads in front of. Advertising in the US was more than half of Google's revenue then. Comcast had the only network nationally capable of reliably delivering video and video ads, which was prioritized for growth within Google around then.
I think there was a real fear within Google about the leverage Comcast and other national ISP's had over them. TLS wasn't common, and advertisers weren't nearly as interested in mobile audiences in 2010: wireless data use was a fraction of what it is now. A well-capitalized Comcast without the regulatory restrictions of AT&T or Verizon could create existential trouble for Google if they really wanted to. Google Fiber could have been Google's way of showing they'd be willing to play hardball if it came to that. Any positive PR that came from it was a great bonus, but they almost certainly never made money from Google Fiber.