Developers and the whole world waiting for Android N but still 75% users using Kitkat and lower version not Lollipop or not Marshmallow. Why?

Because android is free programming platform.there may be many bugs in marshmallow in which programmerare still yet to fix many problem such as battery problem,wifi conncetion problem,overheating issues.large data consumpiton issues etc,so its may be resaon for users not for upgrading in android 6.

Because manufacturers are lazy and don't wanna update their phone's OS. It's also a commercial tactic plan.