any idea why is google the most popular search engine?

Two reasons really. Early on it just gave better results than everyone else. Ranking pages by the links pointing at them was a big jump over the keyword-matching engines of the time, and the page was fast and clean. Then being the default did the rest. It's built into Chrome and Android, and Google pays Apple and others a lot to be the default search in their browsers. Most people never change a default, so the lead keeps itself going.