Why would the AC only blow cold air when set to LO?

Temperature control concerns can cause temperature blend issues in the dash, which prevent sufficient cold air in the mix due to possible motor or blend door issues. It's just a guess though. You could have a concern where the air conditioning system does not produce enough cold refrigerant to blow air through. Hands on testing is needed, but I'd probably start by ensuring cold refrigerant is circulating, then check on the inside of the vehicle and the temperature blending function. www.repairpal.com/repair-shops