Unity did bet on the mobile market and won big, so no surprise is so popular nowdays. Also it still is very mobile friendly. I am willing to bet however that will quickly feel the heat from Unreal , if it has not already felt it.
By the way its good news that Unity is more popular, that means that Epic developers will try harder to improve Unreal. Being on the top is a guarantee for feature freeze , or slow down to the development, relevant examples are Microsoft and Autodesk. This why countries try to fight monopolies as much as they can.
So I am definitely not eager to see Unreal get the crown. Second place is great place to be 