In almost every area & every country, Google is king of search. So we tend to mostly focus on getting good Google search rankings. Google has about 92% of the search market which means that, even if we ignore the all the rest, we will still cover the vast majority of Internet users.
Having said that, there are still sometimes good reasons to include on other search engines. For example, Bing is Microsoft's search engine & they also run Yahoo and AOL, so if we are only focused on Desktop users, then looking at Bing as well might make sense.