I don't think fraud is a driving cause here.

The real cause is downgrade protection. When your browser loads an HTTPS page, it will refuse to load (or at least warn when loading) any other resources over HTTP: js, css, iframes, etc.

This is to ensure that the icon you see in your URL bar is actually accurate: if a page loads over HTTPS, but consists entirely of a single HTTP iframe, that nice green lock is totally meaningless.

So when the OP switched their site to HTTPS-only, they lost the ability to ever display any from HTTP-only bidders. So the set of people bidding on their ad slots went down, the price went down, and their revenue went down.