Although it would make sense... it does condradict the idea of "diplomatic" victory, any warmonger is usually hated, so by conquering capitals you really are in a way losing votes...
Diplomacy is often another way of waging war and wars are another tool in the arsenal of a diplomat, and it's true even in Civ. You bribe other civs to attack one another so they won't attack you, you conquer CS through corruption (bribery) and coups, you attack the economy of a rival by plotting behind the scenes to get embargoes passed, or you attempt to destabilize his regime by banning his luxury goods. You form leagues when agreeing to join someone's war to strengthen an alliance. Etc.
There's no reason why diplomatic victory should be blocked to warmongers, if they're good enough warmongers.
In the current form of the game, giving extra delegates to Autocracy would probably be overpowered (nothing stops an Autocrat from bribing enough CS to win, or at least to block another from winning while he finishes his conquests already).
Autocrats should get 1 delegate from captured capitals and 1 per 5 puppets *only* if they remain puppets. They would lose the bonus if they annex those cities into their Empire.
An Order tenet might provide a similar bonus: annexed cities become "satellites" and provide 1 delegate per 2 or 3 puppets from the same civilization.
Freedom could permanently get 1 delegate from another civ by liberating and giving back to its original owner a conquered city. 2 delegates for 3 liberated city, and 3 for 5 cities (from the same civilization). Liberating a city you've conquered yourself or got in a peace deal in the earlier game would count (an incentive to do a bit of "colonizing" in preparation for taking the Freedom ideology).
That would make it more difficult to win diplomatically only by bribing the CS.
Personally I'd much prefer if with Civ 6 the VC were turned into "achievements" anyway. There would remain only Time and Domination as conditions to end a game. Having achieved one or more of the other "VC" would boost the score, and perhaps give a bonus to the player in game when an achievement is reached. They could introduce a bunch of more minor achievements (religious, economic etc.) to complete the picture.
I don't play with any VC conditions disabled though, with the current game I find that too unbalanced for some civs.