It is possible that the H fare is sold out on one (or more) of the segments between MKE-ORD-SFO-NRT-BKK (the usual routing) which would automatically kick you up to M.

Also, UA just might not offer an H fare on the domestic side (MKE-ORD or ORD-SFO) which also would kick you up to M.

UA will make H seats available, however, in case somebody is connecting from another city with available H fares (say SEA-NRT-BKK).