AFAIK there was licencing issues with memristors - in usual forward thinking HP way, they exclusively licenced the tech to someone else, who did no work on it, so it was not ready for the machine.

The IP is also a joint asset of HPE and HP Inc, so that sounds like something I would avoid baking into my product if I was HPE

I recall having read that it was not yet economical to mass produce memristors, which had to compete against modern transistor chips.

So they went ahead with the unified memory architecture, just without the memristor component.

One could argue that this architecture is future-ready, once memristors become economical to produce

Yes, but I think the idea was that The Machine was a constellation of technology projects for memory-focused computers. Although the memristor portion did not live up to their claimed expectations (unfortunate but unsurprising), the overall project of a memory-focused computer continues under the brand of The Machine.

(My outside-in interpretation. Maybe someone from HP can add more concrete detail and insight.)

memristors are unfortunately still vaporware