Because of the various aches and pains!
I'm 50. I haven't been able to sleep on my right hand side since having kids 20+ years ago due to reflux. I also have a bad hip on that side that's been operated on in the past (arthroscopy, not replacement) and is still problematic. And a snoring DH who sleeps on that side of the bed.
This means I sleep on my left hand side mostly. But 18 months ago I developed shoulder impingement in first one, then the other shoulder. This has mostly resolved with physio and paying attention to my posture. BUT because posture is key and slouching the shoulder means it keeps being painful, favouring sleeping on that shoulder means that the pain hasn't resolved on that side and is lingering. It makes me grumpy as my favourite place to be is snuggled in my bed, but it's often quite uncomfortable.
I've ordered a wedge pillow to hopefully get around the reflux issue and mean that I can sleep more on either side.