GB and the US won medals in all relays, so 8 out of 12 medals went to only 2 nations, not much diversity ...
I Jamaica wouldnt have had so much bad luck, they probably would have won 3 of the remaining 4 medals ...
In womens 4x100 m relay there were 5 nations at best that could win a medal (USA, Jamaica, GB, Germany, TTO), but the US was lightyears ahead. In womens 4x400 m relay they were actually only 3 nations that had a chance to medal (USA, GB, Jamaica), but thanks to an injury Poland won a medal there. In both mens relays the diversity is much bigger, although in both relays they were only 2 or 3 relays that had a realistic chance to win.
It is not just the domianance of the US (especially on the womens side), but the fact that, if it wasnt for injuries, 11 out of 12 medals would have gone to just 3 nations and in addition to that the level on the womens side is just atrocious.
In addition to that the medallists dont change much over time, USA, GB and Jamaic have been dominant for a very long time and they basically are always almost a lock for multiple medals in relays. It is just much less likely that a small nation has 4 very good athletes than a small nation having one extraordinary athlete who can win a medal, so there always will be less diversity in relays.