Google started the process of enforcing HTTPS on AdX about 2.5 years ago. Not implementing HTTPS in that large timeframe is rather inexcusable. It's just technical debt that you need to acknowledge, and then get rid of it.
Nowadays an Ad Server can call another one and so one, due to programmatic. So you might have one Ad server enforcing and serving their tags in HTTPS, but in the end the little local Ad Server which is part of a bigger network doesn't have the HTTPS and so his material isn't delivered.