Lots of people in a pretty small area in relatively dense cities that currently drive or fly between the cities (technically called strong city pairings). There's also a pretty enormous tourism industry in Florida that captures much of the Midwestern US/anyone not going to California or Hawaii for their beach or disney vacation. Florida is also flat which makes for very cheap high speed rail. Note how the map goes out of its way to avoid the mountains out West.

That being said, I'm not sure this map is one of the ones made with serious city pairing calculations. I'm skeptical that Quincy, IL has a really strong draw for high speed rail, for example, and that long gap between Portland and Sacramento/San Francisco, while beautiful and filled with cool places, is way too sparsely populated to justify 6hrs on high speed rail. I think it's a sort of meme map that's been going around for years, though I wish it were real.