If these are what "we" think they are, i.e. oxygen failure without the pilot realising it, maybe the problem is that some pilots are flying without monitoring their oxygen levels.
It is probably easy enough to lose oxygen without realising it.
That 737 over Greece managed it perfectly well without either "professional" pilot suspecting a thing, despite cabin altitude warnings going off everywhere.
In a GA cockpit, in an old-style fitted system (as is typically found in turbocharged planes) the cannula or mask just plugs into a bulkhead-style connector / variable-orifice flow restrictor, which is fed with a constant pressure (about 20psi) from a first-stage regulator which is located somewhere near the cylinder in the boot. If you stop sucking the oxygen, there is no warning.
And most turbocharged pilots will just be using the default provided system. I bet most of them will have never heard of anything else. This may be more true of the Cirrus customer base, due to their innovative (not intending this in any sarcastic way) marketing.
With the Mountain High electronic demand reg, if you stop breathing, after a while you get an audio warning, but if you merely breathe lazily (which at 25k won't be good enough) you won't get the warning.
With the Precise Flow demand reg (which I have) you hear only the distinct hiss of the oxygen pulse as you inhale, but there is no warning as such if you stop breathing.
This kind of thing could get any of us, and could catch anybody flying at 25k in one of the old-style fitted oxygen systems unless they watch their blood oxygen level carefully.
The problem is they have no backup - other than a rapid descent which isn't going to be an attractive option if there is nasty stuff below, which is probably why you were at 25k in the first place
I don't have a backup for a first-stage regulator failure for example... it's not really practical.