They keep seeing very popular apps and messaging products that are completely proprietary and locked in, and that are able to move and innovate fast because they either don't have to conform to an existing standard, or worry about publishing and support whatever internal protocol they do use.
Sure Hangouts could publish their protocol like Wave did, but Wave was a complex mess of different protocol formats and transports and publishing that as a "standard" was way premature. Who knows how good the current Hangout protocols are, or how well they might play with real standards like WebRTC. It's probably too early for them
I remain optimistic that these things can be standardized in the future.