Because non-IT users (your customers) are sick and tired of apps from 2000 that run clunky and ugly because someone is "forcing you change it" (aka doing your job) and you don't feel like it. Yes,
I know some extremely well-written apps from 2000 without trendy obfuscating flat interface, and many trendy albeit unusable monstrosities from the past 3 years. What kind of argument did you make?
Embracing pure technical churn is no ones job. That's why Python3 is viewed as a problem by so many. People embrace true technical innovation, which it is not.