My mom has a Windows laptop which she uses without even really understanding what a web browser is, or what a search engine is. She uses Bing because that's what the Windows search bar defaults to and because the default web browser on Windows by default opens with MSN as its home page, and she doesn't understand she has a choice in any of this, and probably wouldn't bother to change anything even if she did understand and knew how to change it.
I strongly suspect that many Bing users are similarly borderline computer-illiterate, and just use Bing because it's the default.
The same probably goes for many Google users, a lot of whom were probably tricked by some software they installed that had a tiny checkmark and small print telling them it was going to install Chrome and make Google their home page.