There was never a big market for it because new memory was not prohibitively expensive in comparison to the cost, risk, and limitations of using old memory in a new server. That is not the case now, so people are looking at the idea again.
I would counter tho that 1) this isn't the first time there's been a memory price/supply crunch, and "I've got a drawer full of last gen memory I can't use" is kinduva IT cliche, and 2) 'more memory' has always been a pain point, especially with industry practices like chipsets only supporting relatively small physical memory relative to address space (e.g. all those Intel LGA775 chipsets that capped at 4 or 8GB). Oh, and 2a) 'faster disk' has always been a pain point...
But, yeah...obviously my impression of things doesn't match market reality.
They seemed to stop making them altogether around when SSDs came out which probably shrunk the market niche right out of viability.