I wager the basic idea is that it would be a sort of RMT - you could buy stuff out of the cash shop and then sell it on the MB for big buck. Or rather, I wager that is the excuse. The most likely actual reason is: So people who want it have to shell out real money.

Sounds dumb, is dumb, but so is the fact of having the gear of recurring events not recur along with the event in the first place - that, too, used to be different IIRC.