ason that a transition metal would "choose" a hexagonal close packed structure over a cubic close packed? Why would one be more likely than the other?...

Is there any reason that a transition metal would "choose" a hexagonal close packed structure over a cubic close packed? Why would one be more likely than the other? I am looking at transition metals that have 7d electrons: Cobalt, Iridium, and Ruthenium. Cobalt and Ruthenium both have a hexagonal close packed solid state structure, (at standard temperature) while Iridium has a cubic close packed. I cannot figure out why! I know that, in general, a close packed structure is the most efficient structure for transition metals, because it uses the space in a way that maximizes electron movement, but why do elements with the same d orbital electron configuration have different structures? Thanks!