Just because it uses UE4 doesn't mean it can be easily ported. There is middleware that needs to be ported and C++ code that needs to be cleaned up because Clang much more picky than MSVC or maybe even rewritten because of OS specific code. The devs will also have to make sure that the rendering APIs used on Linux are on par with the renderers used on Windows and that there are no bugs or shaders that won't work well on OpenGL/Vulkan.
All of this for an OS with very, very low marketshare.
I want Fortnite on Linux too, but you really can't just say "haha its so easy lolll just port it"