I think
@Dwarfer1979 has made a lot of very accurate points. I've made the point before but those of us on these forums are atypical so we're likely to default to public transport. Most people don't, and don't travel for fun. There has to be a defined reason to travel.
The demographics aspect is something that is crucial. People think poorest in society = good bus territory and that's frankly a myth. If you're long term sick/unemployed, you don't travel if you don't have to. Hence why
@Dwarfer1979 makes the distinction about the
working working class. As an aside, those traditional "sink" estates that used to rely on buses for links to banks, post offices or shops... benefits direct to bank accounts is one reason to travel gone but also the biggest of those estates now have their own stores, encouraged by councils/governments to move into the area to provide employment. Places like Hartcliffe (Bristol), Seacroft (Leeds) and Buttershaw (Bradford) now have major supermarkets when they wouldn't before and as well as reducing shopping trips, it means there's local employment so people have less need to travel.
Speed of travel (point 3) is also important. Operators are often criticised when buses make circuitous routes in order to replace another service that wouldn't be viable. It's a balance between having two sub-optimal (even loss making routes) and having a good British compromise. I know enthusiasts love an express service but in many places, there's simply not the passenger base. Moreover in many locations, the issue is that buses are simply stuck in the same traffic as cars. So if you have a car and the parking is cheap (see point 2), why wouldn't you use your car if you have one? However, if you can introduce bus priority and reduce car parking provision, then you might start to affect the balance. Of course, local authorities are dependant on car parking as a valuable revenue stream (so you understand their reticence in reducing that) and putting up parking charges is always a bad news story, as is removing parking from outside local shops and businesses. It's a real challenge for councils, let alone operators.
Point 5 is one we all recognise. I know that Thanet (Westwood Cross) was mentioned in the post that started this discussion but I mentioned Bishop Auckland in Co Durham. It was a traditional thriving town centre but when I was there last month... the place looked like the zombie apocalypse had struck. Instead, the council has allowed a major car centric, out of town development to be built over the last 10-15 years that has abstracted most of the retail business. This is not new - it happened first with Merry Hill, effectively gutting Dudley's high street and that was in the mid 1990s which is why PPG13 (planning guidance) was introduced. Of course, if the shopping centres are large enough, they can establish a critical mass such as Meadowhall, Merry Hill or Bluewater to justify their own routes. Most aren't that big and trying to serve them AND a town centre is often difficult and simply fragments passenger flows.
Instead, it's been the more far sighted councils (like Oxford or Portsmouth) that have sought to curb the rise of out of town shopping centres, limit town/city centre parking, improve bus priority and have good partnerships with operators (who also have a duty in all this - let's not forget) and it's that which is driving bus patronage.
Buses work best when you're moving strong major flows and these are predominantly into the town/city centre from the outskirts. Move from that and fragment the flows and it makes it a lot more difficult.