A more interesting question is why there are any non-tonal languages. From the standpoint of communication in noise, it seems one would want to put as much info

A more interesting question is why there are any non-tonal languages. From the standpoint of communication in noise, it seems one would want to put as much information as possible into the part of the signal with the most power, i.e. vowels, and tones are acoustically a property of vowels. Yet in most languages (as far as I know), consonants are much more informative than vowels.