caused a hit on performance that would be too much for some devices, so Google backed down and made it optional for manufacturers to turn on encryption by default. The new Android Compatibility Definition Document states:The problem in Android 5.0 was that full-disk encryption
You'll notice there is two big caveats in the requirement. The first is the same as when Google tried this with Lollipop - full-disk encryption will only be required for devices that launch with Android 6.0. Devices that get Android 6.0 through an update would be exempt from this requirement. The second one is more important - requirement will also depend on the device meeting a certain performance standard.
Presumably, if a device meets the performance requirement, the hit to performance caused by encryption won't be as noticeable, and if it is done out of the box, users won't know what they're missing because it won't be possible to set up the phone without encryption.
Interestingly, Google is not requiring lockscreen security to be set up along with the full-disk encryption, which would seem to work against the security added by encryption to an extent. We'll have to see how users and manufacturers react to the requirements this time around.