I think a lot of it is due to Windows 10. If you get upgraded from Windows 7/8, you always start with Edge as your new default browser and Bing as search engine

I think a lot of it is due to Windows 10. If you get upgraded from Windows 7/8, you always start with Edge as your new default browser and Bing as search engine for both browser and Cortana. Most people probably haven't switched back.