Ah, so are you suggesting that the injunctions of Mohammad were merely the norm of the times? If so, I agree with you.
However, most Muslims believe that the words of the prophet (in the Quran or Hadiths) are for all eternity.
Instead of "better treatment of slaves", your prophet could have said "Thou shalt not own slaves at all". That slavery (ie, owning another human being) is vile and despicable. We know today, that that is the truth - any human owning another human is a horrible, despicable, condemnable concept - it is a pity that this did not occur to Mister Mohammad.
Sure, it was the norm of the times - but shouldn't the prophet of the great God know better?