Because your shoulder and hips are the primary pressure points on your sleeping surface when you’re straight-up fetal, you need something with a little sink-in squish to keep your spine aligned however, you also need support for your limbs, ribcage, and tuchus. Being a side sleeper is the elite position, but it also presents its own set of criteria when you’re looking for a mattress, pillows, and even bedding.
I’m a lifelong side sleeper and can’t imagine being anything but.