Google added phone data encryption in Android 5.0 Lollipop as a clear way to protect users from potential loss or theft of their valuable device. What initially seemed like a major innovation has ultimately become a nuisance due to performance issues when this encryption is enabled, even on a phone like the powerful Nexus 6.
So it seems Google has quietly revised the Android 5.0 requirements to allow manufacturers to decide whether to enable full device encryption. This ability to have a fully encrypted phone was part of an agreement to provide greater security for users in case their device fell into the wrong hands, so we'll have to wait until these performance issues are resolved before we can implement this feature.
Data encryption by default
This encryption means that new smartphones running Android 5.0 Lollipop will have this option instead of the older encryption found in previous Android versions. When this encryption feature was announced by Google and Apple, it initially caused some controversy, particularly among US law enforcement, who were unable to use a phone with it enabled.
Just in case
Google doesn't want new devices, which may not offer significantly higher performance since they aren't high-end, to have problems with default data encryption, so it's leaving it up to manufacturers to decide whether to activate it at the time of purchase. It's worth noting that this feature initially appeared only on the Nexus 6 and Nexus 9, while other Nexus devices already on the market, such as the 5 and 7, had encryption disabled by default.
At the moment there is no official announcement from Google in this regard, so we will have to wait and see if they comment on this decision to quietly backtrack on full data encryption by default.