I think a combo of these two, and the point about machine-built tracks that are running to a budget (and trying to turn a profit) are the main reason for the mainstreaming of flow trails - or at least the same kind of trails being built so often.
@caad9 the ROI is more about how many people it can attract to the town, given that the trails are mostly free to ride and taxpayer funded. The goal is to attract tourism (alongside the fluffier stuff about encourage physical activity, community, etc.).
I've seen previous surveys of people at trails like Stromlo asking if you're from out of town and if so, if you stay in a hotel, eat out, use local bike shops, fill your car up with fuel, bring the family, etc. The idea is that it attracts a tourist that has disposable cash and that spends money on coffee, hotels, food, beer, parts, fuel, etc. The money invested in trails goes back into the local economy with domestic tourism. So the ROI is wheels measured by the amount of people using the trails. And that's where
@rowdyflat post comes into it - they need trails that appeal to the largest common denominator, and that's people who want to buy a half decent bike and have fun on it straight away - not spend years developing skills and fitness.
For me, the trails don't need to be double black, they just need to be varied and not all the same style of trail. Narooma seems to have taken that approach, which I'm stoked about (have an email train forming as I type this to organise a trip with some blokes in a few weeks - YEW!). My fav trails are ones that really use natural obstacles, have some jank, have some fast sections and some more challenging sections, etc. For those who know the trail down the back of Stromlo - Wedgetail down to the bottom of double dissolution, that would be what I'd like to see more of - fast and that little bit sketchy with shit that you have to properly navigate.