Z998 android stuck on version 4.1.2

A 'check for updates' under 'AT&T Software Update' on a Z998 Gophone always returns with the message 'current software is up to date'!!! 'About Phone' lists the following legal information: Model: Z998 Android Version 4.1.2 Baseband version P893A21B01 Kernel version 3.4.0-svn11840 zte-kernel@Zdroid-SMT Build number Z998V1.0.0B12 I purchased this Z998 phone (branded as AT&T) at Amazon in April 2014, It has 2GB internal storage only. My question: since my software is very much outdated, why can't it be updated, say from Android Jelly Bean 4.1 to Kit Kat or to Lollipop? Thank you for your help!!!