I think it was to do with runway length but when the new generation of prop aicraft came out in 50`s the then Ringway had not been extended ( I think 1957
when runway was extended to 7000ft) i:e DC-7 and Stratocruiser and the various
versions of Connies
Have a look at a copy of First and Foremost the first 50years by RA Schofield
published in 1978. Just checked you can still get it on Amazon a very good read if you are into airfield history
Chaps