The .3mf file format has a lot of advantages over the .stl format. But I believe the reason the .stl file format is still used more is because it is an older format and works natively on a wider range of software. For example, Blender can natively import/export .STLs but not .3MFs That is changing though and more and more programs now work with .3MFs, including PrusaSlicers.