Why are some fields not sortable in a table view?

In particular, I need to be able to sort my Views table by whether or not a node is flagged. In studying the problem, I’ve noticed that some fields can be made click sortable, and others cannot, and I can’t see any common elements between those that can and those that can’t.

In the case of Flags, is it an issue with the module itself, or is it something on the Views end?

Just trying to figure out where I need to start in seeking support. I’d appreciate any information. Thanks so much.