I think there are a number of factors that influence where Emirates hold their open days. If there is a lot of interest online (applicants applying on Emirates Career website) then they are likey to hold an open day there. In the case of Melbourne, maybe there has been little interest in previous months and hence the company has not scheduled an open day.
I have noticed a big recruitment drive in Germany also which could contribute to fewer assessment days in other parts of the world as recruitment teams are tied up there.
I'm sure there will be another open day in Melbourne soon. I very much doubt a QF/EK code share agreement would impact on recruitment in Australia. If anything we may see a recruitment drive in the coming moths as EK tries to 'woo' Australian travelers