Why does NMR deal with protons?
NMR deals with the nucleus of an atom (not just protons, but all nuclear isotopes with spin>0). Electrons also have a magnetic moment, and the technique used to probe this phenomenon is EPR/ESR. While NMR uses frequency in the MHz range (MHz), ESR uses higher energies in GHz range.

Lots of common references available on the internet to learn more, but you might be interested in:


https://physics.stackexchange.com/questions/171256/whats-the-difference-between-nmr-and-epr
https://en.wikipedia.org/wiki/Electron_paramagnetic_resonance
https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance