This is based on observation, not detailed study:
DF's current inheritance structure is wonky. Any children of the current monarch are (potentially) valid heirs - AFAIK there's no preference for elder or younger descendants. Babies and children can't inherit. Also I think that in most cases already holding a position prevents them getting a new one by inheritance - so if they've become mayor in the meantime, they're no longer in line for king. But I've also seen cases where otherwise valid-seeming heirs were just... not chosen, for no reason I could identify.
If the monarch dies without valid heirs, the game may pick a more distant relative of the royal lineage to be the position holder - I've seen dynasties continue through "cadet branches" a few times. But sometimes it just creates a new dwarf and makes him king. I don't know what conditions that, if anything. My guess is that the game recognizes some folks as having a "claim" on the monarchy and elects one of them if it can, but if it can't it doesn't choose a claim-less pre-existing dwarf - it always makes a new one. However, none of these "claims" are visible anywhere in the game's interface (even in exported data), so it's very hard to tell.
Since void dwarves are summoned into existence arbitrarily, they often don't have time to start their own dynasties - they marry too late, have too few kids, and their kids don't have time to grow up or raise their own kids before the monarch dies - so another void dwarf takes over.
All of this will, I'm sure, get fleshed out in a future dev arc, but for now probably the best way to keep dynasties alive is to keep the world relatively safe. A megabeast, goblin, or undead attack on a fortress can kill a monarch's whole immediate family, endangering the succession. You need time and safety for folks to raise families and have several kids reach adulthood.
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Topic: Why do only void dwarves appear as position holders after some time in worldgen? (Read 641 times)