Where have all the postcards gone?

I subscribe to a number of (paper) magazines, delivered by the US Post Office.

Up until recently, I would expect a couple of loose postcard subscription offers to fall out of every issue. And in some cases (the Economist being one), stapled-in postcards that you would tear out if you wanted to use one. (I generally used them as bookmarks).

The last month or so, I’ve noticed that magazines aren’t coming with postcards inside – loose or stapled.

Why is that? Is there a law prohibiting it? Was the cost-per-card not worth the return?

I never understood why they included postcards for people who already were subscribers, anyway.