Timing and precedent.
Unless the BBC has already hired the new Doctor without RTD's involvement and doesn't plan on revealing their identity through a big press event, there just isn't time forRTD to hire the new Doctor and for said actor to film a regeneration reveal as part of the film shoot for the BBC Centenary Special - which is currently underway - and I don't really see the BBC assembling a crew to film said regeneration reveal in a vacuum.
The outgoing and incoming Doctor do not have to be there at the same time or day. Film Jodie bursting and when the new Doctor is ready, watch the video, set up in position and viola, regeneration.