What would make my inside rear tire go bald?

you bought a used tire and overlooked it perhaps?? Very little information given. Also, when the chassis is crooked OR axle is out of adjustment OR rim is sitting on the end right creating a toe in /out. This can happen when you have an older car OR bought it from someone who ran over a curb, it bumps the alignment of things.Tires on those cars don't always sit flat on road and therefore inner wear.. if that's what it is doing. I had a Mercedes that had got in a wreck from the back corner where someone had hit it hard enough to create substansiall interior damage ...enough to bend chassis. Odds are the garage did as much as they could at the lowest possible cost and threw it back in public ( sad). That tire always wore crooked on that axle because of chassis damage was enough to affect how it handled in a crooked fashion YET not be detectable while driving. The bondo on the trunk was in plain sight, however.

Maybe the strut is worn? Have an alignment done, and tell the mechanic that the rear tire is worn so they can focus on that.