Why are tires wearing out on the inside?

I'm assuming your talking about a vehicle or possibly trailor tires.. Case being, Tires that wear on the inside have a negative camber and tires that wear on the outside have a positive camber. If you would notice a lowered car. 9 times out of 10 the tires are leaning in towards the vehicle. That would be your negative camber only a camber kit would resolve. As for positive camber almost the same scenario. Check tie-rods and control arms. If you don't see anything wrong and didn't adjust anything. I'd get it to a shop and have it looked at. If we're talking about a trailer with no weight on it. It is possible for this to happen due to heavier suspension and no weight to balance it.