Young couple goes on van tour of US, boy comes back with van - no girl

why are so many so captivated by this particular homicide(s)?

For me, it's partly the National Park part. I am always attracted to stories involving NPs. The other part for me is that she was attempting to document this whole trip as a vlog so there's so much that's VISUAL about it that's compelling.

why are so many expecting a payoff so quickly, given fed le track record?
Because no one expected him to run in the first place let alone be gone this long.

why do so many believe they're owed some sort of explanation?
It's the American Way.

ETA: Also, the ID Channel is a huge thing.