The Joy of Plucking isn't a guitar playing position guide, but it should be.
There is no doubt whatsoever that physiologically speaking, left thigh, foot raised is where your body will be happiest for longest, long term, and the guitar's positioned to project to a theoretical audience. But most of us rarely have an audience and, like all forms of exercise, the easier ways of doing it are seductively attractive.
Right thigh means it's easier to see and get to the full fretboard, and most of us need a bit of help with seeing what we're doing. But it also means a twisted spine, and those of us who are getting on a bit know that spines need all the help they can get... the more you look after them when young, the less troublesome later life is likely to be.
I rarely play right thigh now. I either sit back on the sofa with the guitar resting between my legs in 'leftish thigh/straight back' mode, or a stand up and play. I've actually grown to prefer standing up unless I'm feeling weary. I, and what I'm playing, feel more 'lively' when I'm on my feet. Purely psychological I'm sure, but my fairly sedentary life needs all the help it can get, so any excuse to get up is a good one.
I suspect plectrum playing is the biggest factor though. When I'm fingerpicking I usually have the guitar in a more classical mode, with the back of the guitar fairly parallel to my body; when strumming or flatpicking, the guitar body's more angled so my arm's moving more naturally up towards my left shoulder the away, down to the right.