Yeah, I see this. Once you reach the cap and a site is lost, another race can "steal" that slot to make another site of their own and so the original race loses the chance to reclaim, because if it was reclaimed the number of sites would go over the cap. And then as history moves on they lose population via attrition and so can't reclaim even when the number of ruins would allow it.
So lets say you allow 4 races in worldgen (we'll ignore kobolds for simplicity's sake) and allow 20 sites. On most seeds you'll get them all starting with 5 sites. Then, depending on the history, if a dwarf site gets razed, they may or may not be able to try a reclaim if they have the
mandwarfpower because another race might take the empty slot to build a site, giving themselves 6, again hitting the cap until some other site is razed. At least that's how I
think it works from my experience. It's tricky to work out what exactly is going on, as you know.
I don't think it is race dependent beyond certain races being more or less likely to lose sites while other races still have capacity for expansion. I find that dwarves almost always lose a fortress early to a (edit, typoed megabeast earlier)
megaforgottenbeast, for instance. The other races don't have that danger, but the dwarves share in all of theirs.
One potential way to get around it is to try and use savagery settings to cut down on the site sprawl rather than the site cap number, but that's hardly perfect either.
Most of the time I gen custom 33x33 islands with 8 megabeast caves and 16 semimegabeast caves, 5 civs and 50 site caps and then just reroll for ages until I find a history (that I like) that keeps all of the civs alive for at least 100 years. Usually takes a lot of regens.
I reserve the right to be utterly wrong about how it all works