Although it’s not a flagship smartphone, but LG still provides the most recent Android OS update for the LG G Stylo, the number of users could be the reason why LG pays more attention to this mid-end device. In the US, this phone has been sold by numerous carriers like T-Mobile, MetroPCS, Sprint, Boost Mobile, Cricket and Virgin Mobile. Almost all variants have updated to Marshmallow, including the T-Mobile LG G Stylo (H631), which has received the Android 6.0 update in early March 2016.
However, even though Android Marshmallow brought many improvements and new features that could attract its users users to update it immediately. But there are still some users are still hesitant to update their phones to the latest version due to technical reason. They are mostly geek users who have managed to get root on earlier versions of Android. When they are very relying on superuser access, they would prefer to stay on Lollipop.
The reason this is acceptable, because it is not easy to get root on a device running Android 6.0. Some people even try their luck by using a one-click app like Kingroot to regain root on T-Mobile LG G Stylo running Marshmallow. But the results are not very good, as it ends up with soft brick.
Those who have the make an attempt to root the phone with Kingroot will find their device stuck in bootloops with code 1003, we do not know what that meant, but it was certainly a bad indicator. If you have this problem, you can try to restore the phone back to stock Lollipop firmware (10L) using LG Flash Tool.
We may have to wait a little longer until a new method to root LG devices (running Android 6.0) can be found. We will let you know when we come across it.
Thanks @paco_1 from xda-developers.com