Why do nuclear protons have an opposite electronic charge to electrons?
Why? Because they have equal and opposite charge to electrons.
Fundamental particles, electrons, and protons, HAVE opposite electronic charges. Historically, electrons were assigned a negative charge; and protons, massive, charged nuclear particles, were assigned a positive charge.
There is nothing fundamental in this assignment. Experimentally it was found that protons and electrons HAD equal BUT OPPOSITE electronic charges. In fact, if we were to make the assignment today, we would assign the electron a POSITIVE charge, and the proton a NEGATIVE charge. Such an assignment would have saved generations of physicists and quantum chemists from getting the wrong sign on their answer simply because they miscounted the number of electrons when they solved a problem.