It's not the marketing. It's the fact that it's what biologists call expensive signaling. You are spending money, not on a trip that you both enjoy, but on a ring that really only she enjoys. This is an expensive (therefore hard to fake) signal that you care about how she feels.

Now, imagine that the diamond is cheaper. It is no longer an expensive signal. Therefore, although the diamond is the same, it no longer serves the same purpose. So, the cheaper diamonds get, the less demand there is from prospective brides, even if only marginally. This fuels further declines, although that is probably already happening due to better industrial diamonds.

This takes time, of course. But that also means it has a lot of momentum. As the price decline continues, and the word of that spreads, it will accelerate.