Engine issues
I seem to recall that in addition to all said before there were issues with the RR Tyne engine and that was just 'another' issue too many for it to be a Viscount replacement;as the new 'Jets' had more sales appeal.
Having said that i do not know if it was ever even considered as a UK 'Orion'.
I suspect the engine issues were the straw they did not need.
Its all in 'Trubbs' book. I forgot it also had some unpleasant stalling issues and was not considered to handle like the Viscount.