Why do people prefer rail over bus? That's a pretty tricky question to untangle. It's largely emotional because consumer preference is not entirely rational, but it's also a question of investment -- many bus systems are poorly funded and so people's perceptions of buses suffers in comparison.
Could you build a high quality bus system that would move the needle in a similar way to a rail investment? Sure.
Could you do it cheaper than rail? Maybe.
But, would politicians be tempted to cheap out, cut lines, driver schedules, etc to save a buck? Absolutely.
And, would travellers view it in the same way they would view rail? No.
You'd be swimming upstream against consumer / traveller preferences which is almost always difficult. And regardless, I don't see any P3's stepping up with plans, feasibility studies, MOUs, and investment dollars to make a multi-billion dollar investment in a bus transit system.
Two seconds of googling here, but I found a study from a pair of researchers who looked into the question - Bus or Rail: An Approach to Explain the Psychological Rail Factor (2012). Here's the abstract.
https://citeseerx.ist.psu.edu/viewdo...=rep1&type=pdf
* worth pointing out Germany has traditionally been a preferred market for Alberta's tourism agencies to attract international travellers so the German mindset shown in this dataset is particularly useful to understand.