The OHA is a group of companies committed to Android�Google's Android�and members are contractually prohibited from building non-Google approved devices. That's right, joining the OHA requires a company to sign its life away and promise to not build a device that runs a competing Android fork.
For OEMs, this means they aren't allowed to slowly transition from Google's Android to a fork. The second they ship one device that runs a competing fork, they are given the kiss of death and booted out of the Android family�it must be a clean break. This, by design, makes switching to forked Android a terrifying prospect to any established Android OEM. You must jump off the Google cliff, and there's no going back.