Back pain is very common, more so lower back pain than upper back pain. The explanation really involves body mechanics. It is the lower back that takes the brunt of our every day movement and work. When you bend, twist, or lift something, the stress is on the lower spine.
This isn’t to say that the upper back doesn’t work nor does it not get injured – it’s just that the lower back is the hardest working part and is, therefore, more prone to injury.