I couldn’t find much yet for the reason, so I decided to look more at the bigger picture. The image you are looking at is just for Desktop Browsers, and it’s main competitor was Coc Coc, another Chromium based browsers if you notice in the chart below they also took a huge dive in 2021. A little research shows that that big drop was caused by Google not allowing Google Account services to function properly within other browsers, once fixed… it appears they got some of their users back. I have no evidence yet but assume something similar happened.

Note when it comes to mobile/console, whatever else safari still holds some users. So Google is slowly just beating Apple out of the market there. Coc coc hasn’t released a Browser update since July 2024, so maybe they’ll launch an update that fixes whatever broke and make another come back… But long term it doesn’t look good for them

Edit: I noticed gpstatistics site apparently doesn’t save the time when you use their save as png option, so for no x coordinates on the graph, on the first graph the 2021 marker is where coc coc hit near 0, then bounced back