This is a very general gloss of keyboard and guitar ergonomics. First of all, the majority of people are right handed, and historically speaking, in what's known as the common practice period i.e Classical Music, you would want your most skillful hand to be able to play and add decorations to the melody of whatever you were playing. Since the melody is always going to be higher than the bass, it makes sense to have the low notes to the left, and the right hand covering the higher pitched notes to the right. With guitars, your fingers are much more dexterous than your thumb. So you want the higher pitched melody notes where your fingers can get at them, leaving the thumb up at the top to cope with the bass lines. There's no reason, per se, why they they couldn't be reversed. But since the majority of people ARE right handed, this is by far the most comfortable arrangement of notes for the majority of players.