You could ask the air ambulance operator directly - I don't see why they shouldn't be happy to explain if you have a legitimate interest.
Someone else with local knowledge may choose to offer an explanation but, if it helps, I suspect it's nothing to do with ATC or the airspace. You've probably half answered your question already - what is acceptable or preferred in a life-threatening situation if often different to normal operations. Things like noise nuisance, outcome of risk assessment, turnround times and disruption to hospital activities are just some of the factors that come to mind. FWIW, if I were a betting man I would put my money on the last of these.