Why only Google matters?
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.

Source:
StatCounter Global Stats - Search Engine Market Share
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.

Source:
StatCounter Global Stats - Search Engine Market Share
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