Because every feature takes time to create. There are loads of feature in Opus that aren't in those other file managers. Different programs/companies/authors/users view different features with different priorities.
It's on GPSoftware's feature-request list, as you know from the other thread, but as I say in that thread, they haven't indicated when it will happen.
If it's an absolute must-have for you then Opus won't satisfy your needs at the moment.