The top scored is just the answer liked best. The fact that it refers to proton decay and quantum gravity, both hypotheses which, as plausible as they might be, are not experimentally testable at this time, renders in my mind the confidence of the answer questionable.
The top answer has multiple reasons. The one I am referring to in particular is this section: "I should point out that if you believe that the standard model matter is complete, then anomaly cancellation requires that the charge of the proton is equal to the charge of the positron, because there is instanton mediated proton decay as discovered by t'Hooft, and this is something we might concievable soon observe in accelerators. So in order to make the charge of the proton slightly different from the electron, you can't modify parameters in the standard model, you need to add a heck of a lot of unobserved nearly massless fermions with tiny U(1) charge." It makes no reference to quantum gravity.
