Why is there handedness on stringed instruments? If you're right handed, you strum a guitar with your right hand, fret with the left. But both are fine motor skills requiring coordination. Left handed players go out of their way to acommodate doing it the other way around; either finding left-handed instruments (challenge: most things they might like/want are not available in lefty version) or suffer with a flipped right handed instrument (control positions on electrics are wrong, asymmetric cutaways for reach are wrong, nuts are slotted wrong for a mirrored string order).
Strumming/picking requires significantly more (and more frequent) motor control than fretting. Most music will only use a handful of notes/chords, but multiple strumming/picking patterns which need to be performed on precise timings. Your strumming hand is often doing an order of magnitude more precise motions than your fretting hand.
That provides an explanation only in the following way: beginner guitarists tend to strum more while holding fretted positions. While that goes away for advanced guitar, which requires considerable left hand technique (e.g. legato playing), every guitarist passes through a beginner stage. The beginner stage steers right handed players toward using the right hand for strumming. Only those who find that extremely awkward struggle against it and adjust their instrument so that they strum with the left.
Flat-picked guitar solos (bluegrass, metal, country, ...) keep the left hand as busy as the right, and at times more busy. The left hand can easily outpace the right, which is why there are techniques like triplet 16th notes: playing 24 notes in a 4/4 time measure, while picking as if there were only 16.
Guitar playing is ambidextrous, like piano, at the higher levels of mastery.
But at the beginner stage, like learning to strum chords, that's where right-handedness favors using the right hand for activating the string. That's probably what it is.
It can be learned both ways; e.g. Michael Angelo Batio.
Strength perhaps? I cannot imagine strumming very long with my right hand/arm. I imagine it would get quickly sore. Learning to use a fretboard is hard enough with any hand, might as well use the weakest.
