I know for a fact that you can cast delete votes (as you successfully helped to delete a question).
I guess that what you see is actually a side effect of intentional restrictions.
Namely, there are some restrictions on which posts you can vote to delete and the number of your delete votes is limited.
In particular, you can only vote to deleted questions that are closed since some time (two days, if I recall correctly). For users with 20k+ the restrictions are different.
Thus for recently closed questions it is intentional that you cannot vote to delete,
while you might see pending delete votes on some of them.
Of course I might be wrong with my theory. To test this, please, go to some long closed question and see if you do see the delete option there. Possibly wait for the start if a new day to see if it's the case that you are out of votes.
Otherwise please give some specific examples of questions where you cannot vote to delete.