Around 2006, as the suits finally started realizing that normies are in fact a viable market to exploit.
Before that internet and computer use was still fairly niche and you still needed the designated family teenager to troubleshoot everything.
Companies started catering to people of sub triple digit IQ, and it was necessary to streamline things as much as possible.
When mobile users became a sizeable portion of internet users that was basically the final nail in the coffin, as everyone started trying to catch that mobile market and things had to be dumbed down an order of magnitude further.