The original dream of Quietways was there were lots of quiet back streets ready to be linked together quickly and cheaply into new routes with minimal interventions and no fuss.
It very quickly became clear that (a) no there aren't (b) any vaguely direct route was also a popular rat run requiring lots of filtering and (c) filtering was extremely politically controversial and the boroughs weren't interested. Very few were implemented.
The fundamental problem was that they weren't thinking about LTNs, only about the linear cycle routes. They've now found that it's much easier to build a political coalition in support of LTNs for their own sake than it is to demand neighbourhoods be re-engineered for the benefit of cyclists.
(though the interventions in Islington on Quietway 2 have been prioritised partly because they want to improve Q2, and partly because they've been sitting on the drawing board ready to go for years, with political leadership constantly noping them for fear of backlash)