Why downlink frequency is higher than uplink frequency?

Answer 1

Just a case of expediency, started in the older Ham radio and cordless phone era when repeaters came in to use. In a duplex system (cordless and cell phones for example) two frequency's are used in order to have real time both way communication. This got us away from the simplex method of saying "over" to indicate to the other party that it was their turn to talk. Really, it is arbitrary which is the upper or lower frequency, they just have to be different and there are regulations regarding the band width between them.