Electrons

 - have a much bigger charge density as the protons (and especially lead nuclei),
 - aren't compound particles as the protons (and especially lead nuclei)
 - are able to get a much bigger energy with the same fields as the protons (and especially the lead nuclei).

Why seems common for the big colliders after the LEP to use protons (and much bigger nuclei)?

Reacting comments: Yes, to get good experimental data about quark matter they need a lot of hot quarks (= collided big nuclei). But to create new particles, the *energy per degree of freedom* needs to be maximized, and this maximum is at the single particle with the highest charge density, and this is the electron.