@TerminalAutism @ryo blame Google and Jonathan Ive.

Several things happened in 2013 or so: Scott Forstall (the guy behind Aqua and that iconic design language along with iOS) was pushed out of Apple and Ive (the hardware designer) was made responsible. This led to the drastic design language shift of iOS 7, which was divisive but instantly became the trendy style because it's apple and was different and new.

One year later Google pushed out Material Design to Android that did the same thing and the rest is history. This was more dramatic on the Android front because Android 5 on new phones was vastly faster than the older versions thanks to finally forcing phones to use ART as opposed to dalvik so it really made a big splash.