The aircraft involved, http://aviation-safety.net/database/record.php?id=20090601-0 was built in 2005. The wiki tells me flash drives first became available in 2000.

Perhaps my question is why aren't today's aircraft built with this sort of much more redundant system?

Aircraft already have redundant hydraulics, redundant data, flight controllers, fuel tanks, engines, wheels, pilots, .... Why not have "inexpensive" redundant data recorders?

Doesn't matter. They're able to retrofit other things (from seats to in-flight entertainment systems) because aircraft have relatively modular designs and because individual subsystems may need to be replaced or upgraded for safety reasons.

This kind of technology is already deployed on a limited basis in a lot of commercial aircraft. An increasingly common business arrangement is for the airline to buy the aircraft but lease the engines (which are the parts requiring the most frequent maintenance). Rolls-Royce has a global infrastructure for telemetry data designed to shorten maintenance cycles and increase safety: see http://www.flightglobal.com/articles/2009/12/09/336006/rr-tr... and http://en.wikipedia.org/wiki/Aircraft_Communications_Address... for background.