Because the fact is that he is not the mediator for all. Jesus is the mediator of the new covenant for the 144,00 only. Not a GB doctrine, but a biblical one.
Prologos said "The new covenant mediated by Jesus covers all Christians, not just the 144 ooo sealed ones that COME OUT OF the twelve tribes."
Well, yes, the new covenant covers all Christians, but that does not mean Jesus mediates it with all Christians.