The only assumption that can be made is that these recordings are not considered important enough to spend time and money on.
Ian, it's not just the flight data recorders - most of the avionics on an aircraft are several generations behind the electronics in your phone. The reasons are simple - it's insanely expensive to certify new aircraft avionics, and more critical the system, the more it costs (the certification costs to change something in FADEC software is north of a $1 million USD - and that's just the certification costs - it doesn't include actually making the s/w change). Further, if you 'upgrade' something you need to make sure it's a drop in replacement - otherwise all the systems it talks to have to change as well. That's why most 747-400s are flying around with the same flight deck display system that was designed 30 years ago.
Further, while there are occasional 'block' changes of avionics, unless the regulators mandate it, they are seldom retrofit. As a result, there are still lots of commercial aircraft out there that still have the old analog foil tape recorders. I was involved in investigating a 747-200 freighter crash about 5 years ago that was the result of multiple engine failures after takeoff - foil tape FDR, the
only engine parameter was EPR
I've looked at dozens of DFDR files over the years - usually incident investigations but occasionally accident investigations. Trust me, having something like the DFDR in an A330 or 777 is wonderful - they are head and shoulders better than the stuff from even 10 years earlier. Yes, they have their limitations but they are not that difficult to work with once you understand those limitations.