If i remember correctly Feynman said in one of his lectures that we know the mass of the electron with much greater precision than the proton, which may mean that it electrons are easier to study. I don't know if this is still true though.

If it was possible to build a direct neutron accelerator/collider, I suspect we'd get some new physics pretty quickly.

Analysing hand-me-down neutron events from indirect collisions isn't quite as useful.