I think this was a Godfather-style rubout...that is, OBL had outlived his usefulness to Pakistan, so they were ready to get rid of him, while perhaps the CIA (which is about to get a new head) determined that they had gotten all the information they could get from tracking him and Obama thought this would be a good time for him to get a popularity boost by getting rid of a no-longer useful jihadi.
This is not to put down the skills or courage of the Seals who took him out, nor is it to minimize the work done by the CIA. But in terms of why now, that is, why Pakistan dumped him after years of protecting him and why we decided to go ahead with it, I think these are some possible answers.
Said the same thing to Mrs Rainman last night.
Something to get the press off the BC issue.
Something to get the country feeling good about itself and forget the economy.
Something to lower gas prices.
Something to kick off the re-election campaign.
OBL outlived his usefulness ... it was now time for him to give his all to the Muzzie in the WH.