It's easy in retrospect to say that that should be mandatory, but how often does something like this happen? I bet there are a lot of cheap things we could do to mitigate extremely unlikely tragedies.
http://en.wikipedia.org/wiki/Air_France_Flight_447
So, we're talking about more than once per decade.
There is some value in knowing early where an accident occurred, but that is rarely a problem. How much is it worth to fix it for the few cases that happen in the middle of nowhere?
"While Brazilian Navy authorities were able to remove the first major wreckage and two bodies from the sea within five days of the accident, initial investigation was hampered because the aircraft's black boxes were not recovered from the ocean floor until May 2011, nearly two years later"
If they knew where the plane went down, it wouldn't have taken 2 years to find the flight data recorders. The "pingers" on the recorders lasted for about 30 days, which should be plenty of time, if you know where to look.