This is surely a continuation of the trend that we've seen in recent years with shorter, overland domestic routes disappearing one by one by one - some along with the airlines that flew them. Loads of routes have gone - to name but a few:
Edinburgh to Inverness
Leeds to Bristol, Edinburgh, Aberdeen, Glasgow, Exeter
Newcastle to Stansted, Gatwick, Birmingham
Manchester to Glasgow, Newcastle, Stansted, Gatwick and now Norwich
Norwich to Exeter
Plymouth and Blackpool in their entirety
Double APD on domestic return flights, increased hassle and queuing times at airport security and, in some cases, improving rail services all make for a pretty unhappy picture for UK mainland routes. Norwich-Manchester just gets added to that very long list, of which I am sure I will only have covered a fraction above. The surprise is that it's lasted far longer than many of the others, not that it has now gone, I'd say.