I won't pretend that I know anything about the 1980 oil spill in Mexico, but with the BP disaster that we're having now, a lot of the measure failed precisely because the operations had to be done at 5000 ft below sea level. At those levels, water pressure is simply too great, and it both interferes and limits what can be done.
With shallower drills, at least if accidents were to occur, the ease (and therefore success) of repair is much greater. The environment risks at stake is thus much smaller, and the risk-benefit analysis becomes a lot more favourable.
-Lik |