what is reason behind android update application What things responsible for android update. . Means if i have a android phone which will not upgrade my os version .
In this scenario , why it is not upgradable ?.. what factor make it .? 
Any hardware problem or 
any other ?
One of those reasons:


Manufacturer of your phone is too lazy to issue updated compatible ROM with your phone.
Your phone has old hardware that may not work with new android versions, or it's very hard to modify it to become compatible.
You played with your original ROM, that is some companies checks if you are rooted, or if you reflashed new ROMs on your phone, and consider it as you don't deserve to update your phone :D