Why are M dwarf stars the most active (in terms of starspots and faculae) of all stars?
The magnetic activity is driven by a dynamo that relies on a combination of rotation and convection.
M dwarfs have large convection zones or are fully convective beyond spectral type M4. They also have much longer spin-down timescales than G- and K-dwarfs; billions of years rather than tens of millions of years.
This means that a large fraction of cool M dwarfs are rotating fast and have deep convection zones.
This is the basic reason, but it is still a very busy research area.