My guess is that this is similar to StackOverflow switching to HTTPS [1]. Once you get to a certain size, something seemingly simple like turning on HTTPS or detecting desktops and redirecting comes with non-trivial problems. I can only imagine all the nuances that have to be addressed when you're serving the fifth largest site in the world [2].

[1] https://nickcraver.com/blog/2017/05/22/https-on-stack-overfl...

[2] http://www.alexa.com/topsites