The simple answer relevant to this site is that it's a network restriction.
Comments disabled on deleted / locked posts / reviews
Moderators can comment on deleted posts. Relevant MSE post. However, I would use this only very sparingly: there are very few reasons to contact a user after they've deleted a post.
The possibility of extending that commenting ability to 10k users was raised in 2012 but it doesn't appear to have got anywhere. I copied the "disabled" line from a site where I have 10k rep but not 20k. As you do have 20k on ELU, I guess it hasn't been extended to 20k users either.
MSE is probably the best place to raise this particular issue.