Odd question, odd answer :
Obsolescence
By the way, if you wish to ask for support for unsupported solutions good luck.
Here is the official Microsoft post on SQL Server versions supported.
Another reason, is the inter-compatibility between application versions and maintenance cost.
By the way, updating products allow flaws to be fixed, security flaws for example.
Countless the performance upgrades, etc…
To make short : upgrade your products !
Here is the list of features for SQL 2016.
And what’s new for SQL 2016.
Here the list of features for SQL 2014.
Here the list of features for SQL 2012.
Here the list of features for SQL 2008 R2.
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solved Why we need advanced versions in SQL Server and explain specific features it included with simple example