Just some ideas:

- Drone-bombing an embassy in downtown London does not look good on social media

- He's too famous and has many supporters in the Western world to be publicly assassinated, regardless of location (example: Lady Gaga visited him while he was stuck in the embassy)

- He's more useful as a deterrent, i.e., "see what might happen to you", to the people who might decide to go a similar route. Some will go that route regardless, but chances are at least a few have been persuaded otherwise.

For all the ridicule of the government, the Intelligence Community seems to be doing a fairly intelligent job most of the time to satisfy its objectives.