Top reasons I now use bing just as much as google:
#1 Google has increasingly removed results, blacklisted some torrent search sites, and otherwise hinders the ability to search for torrents. Bing delivers top results and indexed torrent sites.
#2 Google censors both images and video's to filter out certain types of adult content, even within adult searches. Content that is otherwise legal yet distasteful. Bing seemingly does this a WHOLE LOT LESS than that of google.
#3 google will censor content based on its "offensiveness" and is coming out with a new algorithm to "determine" what is "fact" and what is "not fact" and will deliver results based on what IT BELIEVES to be fact (https://jonrappoport.wordpress.com/2015/03/01/breaking-google-gives-new-meaning-to-orwellian/
). Bing has no such algorithm as of this point.
I imagine the more google makes these kinds of moves, the more people will use the alternatives.
A monopoly can only exist as long as the people actually want the product or service being offered, when they don't want your product or service, you lose the market no matter how big you think you are.