It has nothing to do with the act of sucking on a mother's breast. It's the stage of an infant babbling: syllables with the vowel a (low unrounded mid vowel) and the consonants b/p/m (bilabial plosives) are the first syllables infants produce at this stage, because these are the easiest consonants and vowels to produce.

And the proto-world hypothesis of mama and papa-words is definitely false; for a detailed argument see Larry Trasks essay on "Where do mama/papa words come from?": http://web.archive.org/web/20051128091658/http://www.sussex.ac.uk/linguistics/documents/where_do_mama2.pdf .

mama/papa-words follow the same laws as all other words: they change their phonetic shape until they are unrecognizable, and they change their meaning or simply get lost and replaced by other words. Words that look like mama/papa are simply being recreated all the time.