statistics....
"Statistically speaking, the number of people who upgrade their 5.x Android to 6.x is almost zero".
Realistically speaking people can't upgrade because the phone manufacturer won't release the new version OS and even if they do, the phone provider will ensure it get buried...
So that if you want to get the new Shiny OS you got to renew the contract and get a new Shiny Phone...
I bought myself a Nexus 5 and albeit it's hasn't all the bells and whistles I had in my Samsung, I am kept up to date really often, with all the security advantages....
And once this phone will die, I won't surely go back to Sammy ....