UE4 has always pushed high-end rendering tech hard and not prioritised mobile. There’s little awareness of the current state of its mobile capabilities, with few big UE4 hits on the App Store (and hundreds of Unity success stories including Pokemon Go and Hearthstone)

F2P mobile dev is also very UI heavy, and UI work with Unity/C# is easier than dealing with C++/Unreal.

And Unity has a wealth of ready made plugins that can be quickly integrated into a project for things like IAPs, advertising, analytics, etc. Although maybe UE is catching up in that area by now?