Only speaking with knowledge of their first market (Kansas City), I would guess it is a combination of three things:
* Buildout costs
Google was using the same contracts as everyone else for the fiber infrastructure install. The guys running the trenchers and directional drills where the same guys that do it for AT&T, Time Warner, Comcast, whomever. Google built at such a pace they certainly were not getting any deals, and very likely significantly increased the costs due to the massive amount of work that was being generated vs. available contractors. Capex cost is everything and they were just blindly writing checks to other people to get it done.
Google was innovative in their "fiberhood" signup process where they heavily marketed to their targeted expansion areas to encourage a certain take rate before build out. I suspect this saved them some money (or rather allowed their CapEx to be spread over more subs), but after that initial signup, I'm not sure there were any efforts to promote further subs. (Actually I'm not even sure people could subscribe after the initial buildout).
There was no Google magic about the fiber installs. It is stringing fiber along poles or burying conduit and running fiber through it. If there was some breakthrough that was able to be made here or technology which gave them an advantage on the CapEx required, it would have been huge boon. There wasn't.
* Access or cost of access to AT&T/KCPL utility poles
Unsurprisingly those that owned some of the utility poles here (AT&T and KCPL) were more than happy to smile while screwing Google's fiber effort. The rates and access rules for these things are set at the Federal level and lobbying by telcos/utilities is fierce. Google's blank regulatory check from the cities was worthless on this front (except for parts of KCK where the local government owned some poles). The cost and regulatory burden for utility pole access is drafted to simply make use impossible or uneconomic.
* A gradual mindset transition to a "Wireless is easier than all this ditch-digging" mindset.
Google has been doing a lot of work with licensed spectrum. Maybe they ran the numbers and figure it is cheaper to buy some spectrum and see if they can work some wireless magic (either technology or special FCC regulation perks) and make it their special sauce. I don't think I'm betting on that particular horse.