Easy for an adult, not so easy for a baby. Bilabial and coronal sounds don't take much tongue dexterity: you just need to be able to open/close your lips and leave your tongue in its natural resting position respectively to produce them. Velar sounds, on the other hand, require you to flex the middle of your tongue to touch your palate, which is as significantly more difficult sound to produce.
It's no coincidence that sounds like 'gaga' and 'kaka' tend to be associated with either more distant relatives or with defecation: the kid's going to be older when those things get introduced.
Not really, no. 'Gaga' or 'kaka' are just as easy yet aren't universal. The mystery is not solved. (Could be just areal effects, for example.)