In response to your question, one of the reasons is that Google basically had a huge head start on search. Google started in 1999 with regards to search and Bing started 6 years later. Another reason is that generally Google provides better and more convenient search tools. One example of this is using and working with Google Search Console compared to Bing WebMaster Tools.
Another reason, in my humble opinion is that Google rules the search world because it also offers so many different products and services that users enjoy like Gmail, YouTube Calendar and Maps. These products that Google offers really go hand in hand with their main service.....search.