A big part of quantum mechanics is the fact that matter can show wave-like behaviour, which sort of breaks a bunch of "rules" that we have from classical physics. This only is relevant if we're looking at stuff at a teensy tiny scale.

Someone else has already mentioned that electrons are a fair bit smaller than protons and neutrons (around 1840 times smaller) and this means they tend to have a smaller momentum than protons or neutrons, which means they have a larger wavelength, which was easier to measure experimentally. That's likely why electrons were a part of this theory, because they're small enough that they're sort of a perfect way to study the idea of things that are both particle and wave, but also neither. In 1940, quantum mechanics and particle physics were super rapidly moving fields, where our knowledge hadn't congealed much yet. What was clear was that electrons get up to some absolute nonsense behaviour that broke our understanding of how the world worked.

I like the results of some of the worked examples here: https://www.chemteam.info/Electrons/deBroglie-Equation.html , especially the one where they work out what the wavelength of a baseball would be (because that too, could theoretically act like a wave, it would just have an impossibly small wavelength)

TL;DR: electrons are smaller than protons/neutrons Smaller = larger wavelength Larger wavelength = easier to make experiments to see wave-like behaviour from the particle Therefore electrons were useful in figuring out how the heck a particle can have a wavelength and act like a wave