How about this for a hypothesis?
If you captured UBL, would you make a public statement about it? Probably not.
A live UBL would be a temporary victory, but a long term problem. The problem with a captured UBL is what are you going to do with him? Have him rot at Gitmo? Bring him to trial? If we are scared of trying the small fry at Gitmo, can you imagine the risk of trying UBL in court?)
No. Publicly capturing UBL is more trouble then it is worth. While there would not be an attempt to rescue UBL, the possibility of AQ holding hostages demanding his release would be very high. Not a good position. So what is the alternative?
What if we captured UBL and not make it public?
Our special forces have many non-lethal tools that allow them to capture someone alive if they want.
What if we captured UBL alive and just announce that he is dead? A burial at sea ensures that the body is never found. To the world he is dead. No one looks for a dead person. No one holds hostages for the release of a man buried at sea.
Once the "body" is wrapped, it is easy to keep the secret from the rest of the crew. Something was buried at sea. The only people in the know would be a small number.
We can then transport UBL to a secure location for what is euphemistically called destructive interrogation. Everyone thinks he is dead, no rescue attempts, no hostage situations, no blackmail operations to force us to release him. We have all the time in the world. After the interrogation, his body is disposed of secretly. A very clean operation.
I have no evidence to believe this is what happened. But it is an interesting hypothesis to consider.