It makes you wonder if theres a bit of inter-museum politics going on here. Some thing along the lines of 'The IWM have an out-station at Duxford and we can't possibly be seen to be copying them.'
But as Duxford is such a success I would have though that a valid reason for the Sciene Museum to follow their lead, learn from the IWMs experience etc.
It all comes down to cost and a failure to be imaginative.