Because it's not something Google is supporting so each manufacturer needs to decide to implement it and on which devices, in addition to actually writing the code. There's a reason Google stopped supporting the feature - SD cards are slower than internal storage, and less reliable. Also - you don't save as much space as you think you do, there's still part of the app on internal storage.
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See
this link for instructions on how to join Android Central.