Are we absolutely certain they did not see it? If pre-occupied with an 'overspeed', they would be cognizant of it, but, like Altitude, they didn't remark on it till it went "critical"?
One would like to think a crew would begin to get concerned when they had lost as little as a 1000' below cleared level -- Attitude Instrument Flying and all that...
Agreed the instrument scan seems to have gone out the window.
Since no mention of altitude was made until 9000' the simplest explanation is that altitude was not in anybody's scan. Otherwise it would most likely have been mentioned by one of the crew much earlier.