The simplest answer is that if you see an answer with delete votes and no comment, then add a comment. If you cast a delete vote, add a comment.
I figured that, since I was the first one to vote, someone else would leave a comment afterward.
Not necessarily. The only thing that you can control on the stack is the actions that you take. There are several users who review a lot of posts and are fairly active in moderating the stack, but who don't often leave comments.
Commenting is exhausting
Now, I know that sometimes it can be a lot of work to write comments explaining why an answer is being deleted. It is especially difficult when the reason for deletion is a lack of backup. I myself sometimes take half an hour or more to try and get the comment right because it is a major pain point for new users.
I've been in the situation many times of wanting to take action on an answer, but feeling like a comment is just too much to handle. What I've tried to do is expand my capacity for commenting. Sometimes when I feel too tired to write a comment, I make myself do it anyway. The more of them I write, the easier it becomes. Don't go too far and burn yourself out. I would encourage you to give it a try and see if you can increase your mental capacity for leaving comments.
When you just can't comment, because I know there will be times where that happens, you can always come to chat and drop a link to the post and ask if someone else could leave a comment.