Check out figure four (4) on this page ->
http://www.clas.ufl.edu/users/rrusso/florida_eq.html
Obviously that oil field was under tremendous pressure when that oil rig tapped into it. My guess is either they CAN'T contain that much pressure to cap off the well. Or the entire area is no so unstable that leaks are coming up everywhere and they are afraid of what might happen if they put the cork back in and the pressure spikes.
Evidently there was a 6.0 earthquake in the Gulf back in September of 2006. Anyone think setting off a nuke (yes, I have seen suggestions to do just that!) in that compressed and stressed strata is going to be a real good idea? Why didn't the engineers know about the stress that area is under after Katrina dumped all the sediment into the Gulf? It probably was like drilling a hole into a soda can after you shook it up for several minutes.