A couple theories I’ve read about more wear on the left side of the tire:
1. Roads are usually “crowned” in the middle, so that water runs off to the sides. If you’re in a country that drives on the right side of the road, when you’re riding straight, you’re actually riding more on the left side of the tire.
2. In a country where you drive on the right side of the road, left turns are longer than right turns (far lane to far lane, opposed to near lane to near lane), so you’ll have more wear on the left side.
If you Google search “motorcycle tire wear left side” this is actually pretty common. But that tire should’ve been replaced a while ago.