It must be that the right hemishere of the brain, is better suited to drawing from a musicians analysis, as to what chords will work, and imagination, as to what notes will evoke feeling in a melody or an improvisation, and expressing them.

Since the right hemishere controls the left hand, you fret the chords and notes with the left hand.

The left side of the brain, is better suited to the more practical, logical, need to stay in time with the music and with other  musicians, so the strumming and picking are done with the right hand.

It must all be reversed for left handers or they would play the same way.

My guess would be that the left hand plays according to patterns, i.e. chords, scales etc. While the right hand has to be more flexible with rhythm and which strings to hit. So RH has less to do, but what it does is maybe more complicated.

My second guess would be that it's a self-fulfilling prophecy. It's true because that's what players are told when they are learning. At the start everything is unco-ordinated and whichever way you start (and practise) that's what will work best for you.

My partner plays the fiddle and I asked her this some time back. She said that regardless of appearances the most difficult part of playing the fiddle is the bowing. The tone and character and indeed the entire expressiveness of the instrument is determined by the bowing, not the fingering. When I've sat in fiddle workshops with her I can confirm that they spend hours talking about bowing and very little time talking about fingering.

Chris

I've been playing the guitar since my 10th birthday, and I'm 34 in a few weeks time. Based on my own personal experience, although the left hand may appear to be doing more, its the right hand that needs the most control. The right hand controls the dynamics of the piece you are playing, whereas the left just controls the pitch of the note you're playing. You use equal force to fret a note with your left hand regardless of whether you're playing loudly or quietly. Because the right hand controls dynamics, you need a sublety and "touch" in your right hand which you don't need in the left hand. You have to be able to pluck the strings accurately with varying degrees of force depending on the piece you're playing. I have a left handed friend who has played guitar for nearly 20 years, has always played right handed, and he complains that he doesn't have any "touch" in his right hand, so he struggles to play gently.   The right hand has to keep time, which is easier to do with your dominant hand (as seen by the way drummers play their hi-hat with the stick in their dominant hand). Additionally, if you're right handed your right wrist is more dexterous than your left, which helps when playing complex rhythms.

It seems to be a common misconception among non players that "left-hand tecnique" is harder than "right-hand tecnique."  I have been a guitar player and teacher for over 30 years and I can tell you that the demands on the right hand are much greater in terms of rythmic consistency, critical angles of striking the strings, accuracy in hitting the correct string, all with split-second timing.  All the left-hand is required to do is form a series of shapes.  It is possible to play many songs using only a few simple shapes, for instance it is possible to play the melody to "ghost riders in the sky" across 3 or 4 simple chords, while the right hand is striking individual strings hundreds of times a minute.  I have found without exception that my guitar students have far more difficulty with right hand than left hand tecnique, especially aquiring rythmic consistency.