They never promised high-density. Semiconductor memristors were always fated at staying at a much lower density than the same amount of capacitive memory. And that's before you get into the manufacturing issues and the problem that it loses "data" when read.

Those things where hyped out of nowhere, with lots of blatant lies making into the popular discourse (like that high-density prediction). I don't even know why, because nobody was making any serious bet on them. They are a very interesting design, that may still get some real-world usage (the manufacturing problems are a showstopper right now), but won't ever compete with flash.