Heheh.
It's not electrical so much as perceptual.
When an atom changes charge, it's usually because it loses or gains electrons. Protons tend to just sit around in the nucleus.
When an atom has one more electron than it has protons, it has a
negative charge . . . but it presumably got that way by means of an electron being
added to it . . . but that means its charge
dropped from zero to -1 . . .
People are not used to associating "negative" or a "decrease" with "addition".
But they didn't know about electrons and protons back in Franklin's day, of course, and so he called one positive and the other negative without any knowledge of what was really going on. As it happens, he guessed "wrong".
[edit]I
think that some engineers -- electrical engineers? engineers in the armed forces? -- use the opposite convention, calling electrons positive and protons negative. This means that their currents flow opposite to everyone else's currents!
But then, they use J to signify a current in an equation, whereas everyone else uses I. So they're just kinda weird like that.