As I dont write the rules I can only assume that this was for the reason of the price of human life. To survive ditching at well over 100 knots would be a feat of pure luck whereas surviving an ejection is not realy a problem. Fly the A/C to a safe(?) point to let it ditch or crash land (away from populated areas etc etc) then eject. The chance of survival from ejection far outweighs the chance of survival from attempting to ditch 200 tonnes of metal, And the MoD dont have to retrain another Jet Jock. (I'm sure they could write the A/C off against tax!!)