I think the modern answer, and it was mentioned early on, is because we can. It was retained, or reverted, in a fit of pique.
The then CAS< Sir Peter Harding drve the change to QNH from the very top. Now that was a rare thing with not just a top man but THE top man driving a change through against all opposition. Had he not blotted his copy book it is more than likely that we would still be on QNH. As it was, even before the revolving door had stopped, but th epresses were running reprinting all the old TAPS.
Now, having gone through the whole exercise twice we probably can't afford to do it for a third time. When we have one sqn and one airfield it will become economic to change then.