Remember that just because an aircraft lands at MAN, it doesn't mean the cargo is destined for the local area. Due to lack of slots/space at southern airports, there is no doubt that some of these cargo airlines are carrying cargo that is meant for other areas of the UK. MAN is 5 hours drive from most major places in the UK so as long as there is a requirement for airfreight into the UK from various Asian cities, MAN could well keep these flights.
Korean, Asiana and JAL all operate freighters to London so they are unlikely to need to send freighters into MAN as well.