What are the technical reasons that SMS is not backed up in a full ADB backup? When performing a backup with ADB, I use the command:

adb backup -apk -shared -all


Why doesn't SMS get backed up?

Every guide says to backup SMS separately. It seems like that would be the #1 thing that Google would backup in the cloud because everyone uses it and everyone wants a backup of it.

Any details of why this is the case and if it will ever be fixed would be appreciated.
Basically what adb backup does is in lay mans terms what a.iso file does for the System.It creates an image file this image file is created as backup.So when ever there is an issue/failure the ROM is restored with this image file.

During the adb backup it will have restrictions due to which it does not backup the contacts,SMS.(You might observed this that it also leaves the contact backup as well.It's hard coded in the OS based on the linux on which Android OS is built).

The contacts,SMS are protected so the ADB backup is not able to backup these two.In order to backup even these you need to root the device.After rooting the device you are indirectly removing the protection so after a root Complete Backup is possible when compared to pre-root where 80% is achieved.