The reason Hillary was the only viable candidate was because she was the only one with the brand recognition to have a chance
Many people believe she cheated in the caucuses, and that's how she won the nomination. Bernie was a viable candidate.
Obama didn't have brand recognition when he started, and he had spent little time in the Senate.
"sweet FA of US voters know the first thing about them unless they’re their state governor or senator"
Most presidential candidates are either governors or senators, (which is how I think it should be) but that doesn't mean that they have brand recognition.
We have a 100 senators, and it's quite hard for a junior senator to make a name for themselves. I suspect the average American can only name 5 senators: their 2 plus a few of the big names, McCain, Warren, etc.
We have 55 governors (50 states plus 5 territories). Most people know their governor plus NY and California, and perhaps another one who has done something ridiculous lately that made it into the news. Why would someone in say New Mexico care who the governor is of Maine? This is a HUGE country, and parts of it are thousands and thousands of miles away from other parts of it.
Being a governor or senator doesn't necessarily give name recognition. In a normal election cycle, people learn about the candidates from appearances, speeches, debates, books they've written, etc.