Are there really that many Kinsey 0s? I find that to be incredibly unlikely.
I think most people like to simplify things for themselves. "gay" and "straight" are simpler than the Kinsey scale. And there's also the fact that in a largely heteronormative society, people may not even conceive of being anything other than straight if they are in the "1" area. Which I guess would influence both their responses regarding their self-identification, and any research done on how the human population splits on the scale.
Or to put it another way - I think most people who are borderline 1 will register as 0s, both for self-identification and purposes of research.
Personal anecdote time, because it relates to the topic - I'm pretty much heterosexual. On the Kinsey scale, I think I oscillate somewhere around 1. Sometimes I think I'm closer to 0, sometimes - that I'm closer to 2. But, in practical terms, I'm pretty much straight and pretty much cis.
The only reason I think of myself as bi is that I'm a furry. To avoid TMI-ing anyone, I'll just say that with (mostly online) interactions with furries, I've been less straight than I normally am. I've had same-sex crushes, at the very least. There's also the fact that furs tend to be more open about sexuality, so that invites questioning phases in one's life.
And I'm not a rare exception. I've met MANY men (surprisingly many) who say that they are pretty much 100% straight in their everyday life, even after being introduced to the furry fandom, but somehow lean strongly towards bisexual "as" furries. Those people are very obviously 1s, sometimes 2s on the Kinsey scale. But if they hadn't been exposed to a certain community, they'd act, think and self-identify as pretty much 0s.
I don't know how it is with people who are not into weird hobbies/subcultures, but I'd imagine most people who are 1s will never indicate in any measurable way that they are. They will just default to 0s.
Personally, I think there are *very* few people who are actually 0s or 6s on the Kinsey scale, but they will be overrepresented in any measures taken to count them. Because that's how we work as humans.
EDIT: when I was typing up this post, my wife pointed out that the Kinsey scale is very... vague. To quote her "if a woman is straight but sometimes looks at another woman and goes 'niiiiiice legs', is she a 0 or a 1? It's hard to tell".