Why is Unity the most popular engine?

Indeed it does. And that is a good reason why Epic Games must step up and up the ante with UE4 which means making it better documentation wise, plugins/assets backward compatibility wise, lower the number of bugs making it more robust, increase the number of assets on the marketplace helping developers as much as possible. Otherwise with Unity getting better they would end up losing large software houses that pay the expensive custom licenses and don’t have their own 3D engine in the works