As you might experience if you visit the PNE's decker on display, the deckers we're getting don't perform so well when you have lots of passengers getting on and off all the time. The single narrow stairway is a bottleneck that would make it hard to get up and down.
Deckers and artics are both capacity improvements over a standard coach, but they specialize differently:
The decker's strength is the hugely increased seating - ideal for busy routes that have long runs between stops and long overall trip times. This allows plenty of time for people to get settled and to prepare to exit. However, the decker has only 2 doors, relatively limited manoeuvring room, and less accessible space.
An artic, however, has a large amount of standing space compared to seating, is mostly low floor and has 3 doors. This makes it ideal for routes with lots of people hopping on and off (especially if all door boarding is used), and for fairly short journeys. The large low-floor space is well suited for folks with less mobility.
The 130, 106 and 49 are busy routes that would be/are well served by artics, but would not be a good match for deckers. These routes are local services making many stops with constant passenger turnover, and the 106 has many seniors (in my experience).
Deckers wont work on all our busy routes and so a huge fleet wouldn't help so much. It's possible to create a north American decker better suited to local service (like london has with the two staircases) but I think articulated busses would still do that job better anyway.