Same thing that happened to them using isotopically pure silicon. Or embedded DRAM.
Was worth investing time and effort into assessing and determining if it was worthwhile to take forward...but dumped as an option once the first milestones were reached.
Happens all the time, you don't make progress without reviewing high-risk opportunities but you also can't expect all high-risk opportunities to turn into production worthy assets.
For z-ram last the world heard was that the original z-ram concept was fatally flawed (the inventor admitted this publicly) but the inventor unvieled a z-ram version 2 that supposedly resolved the critical issues. No one has talked about it much since, publicly at least.