There are a whole bunch of words that "dis" or "diss" could be short for: disrespect, disparage, discount, dismiss, discredit, disdain, disallow, dishonor—and if I had a dictionary handy, I could probably find some more. They all describe ways we don't want people to treat us. I'd like to think whoever coined "diss" had all of them in mind.

I think, when people coin, or use, new words, it isn't only for convenience—in this case, not only because it's easier to pronounce one syllable than three. It's also because they want a word that doesn't mean exactly what the older word meant. They want a word that is either more specific, or (in this case) broader than the original. I think "diss" was meant to describe a broader range of attitudes and behaviors than "disrespect" alone would mean.