Because most airshows won't allow it!! With the exception I believe of the ATC.
Dear Airshow Organiser,
If you want to see the Vulcan display at future airshows (yours included) then you will please allow my team to walk the crowd line collecting donations to keep the girl flying. The appearance of this aeroplane brings lots and lots of people to your show who wouldn't otherwise come and allows you to make some money. Think about it.
Yours sincerely, etc etc
It would appear that Pleming is to lateral thinking as brick is to floatation.