Thanks - I'd be interested in your alternative explanations if you're willing to comment here? I know it's never simple, and often even just timing is a factor in markets. Sounds like you've given it a lot of thought though.
It's mainly an intellectual issue. That is: it's a dispute about what is true in development and education.
In the early 20th c., when American progressives and intellectuals were very interested in education, this dispute was explicit. The (nascent) educational establishment reviewed Montessori's works, attended her trainings and published criticisms, and held whole conferences on the problems with the method. She was very popular for about five years or so, but the popularity was grassroots. The intellectuals always thought she was wrong. Just as one example: the intellectual current in the US was to delay it until later for largely Rousseauvian reasons (reading is an adult imposition that children aren't naturally interested in). Montessori became famous in large part because she taught children how to read earlier. (This is mentioned in the review above.)
There were dozens of issues like this. Montessori thought there should be "an education of the senses"—but progressives critiqued this idea as incoherent. Montessori thought there should be specific, synthetically designed learning materials. Progressives thought there should be more natural experiences and fewer, if any, truly curricular "learning materials". Some thought her approach was too rigid, others too anarchic. Despite widespread public popularity, Dewey, his students, the NEA, and many many many others were vocally critical of her method. By 1916 it had all but vanished from the country, and wouldn't come back for over 40 years.
(There are modern versions of every single one of these critiques, but the overall educational scene is also just much less intellectual than it was a hundred years ago, so it's not as visible.)
But the first half of the 20th century is also precisely when the US school system took shape! It became bureaucratized, and US progressive educators themselves flip-flopped between different pedagogical approaches—from "project based" approaches to "efficiency" approaches that were more vocationally directed. By the time Montessorians clawed back some influence in the US, the basic shape of the system was already in place, and the only outlet it could take was as a grassroots movement. Which meant: lots of entrepreneurial women starting small schools in scattershot ways. Which means independent schools. Which means tuition.
The school system in the US is so badly broken that it also affects the nature and costs of independent schools. And Montessori schools are largely independent schools. So they are more expensive and less accessible. There's a very explicit narrative at play in the review above, one that concludes with completely unmerited swipes at Amazon/Day One. A frank look at current and historical school policy dynamics would conclude that the barrier to getting more Montessori education in the US is the public school system, and that philanthropic and entrepreneurial efforts to push Montessori forward are the only things that have kept it going at all.
The idea that Montessori schools are expensive, rare, and inaccessible because Montessori pushed them to be this way is really ridiculous. It's belied not just by the above narrative, which is a better explanation, but by decades of work that she did that was strongly characterized by humanitarian and activist efforts for the very poorest students, by many attempted (and mostly failed) partnerships that she attempted to engage in with any national government would listen, by her work in India during WWII (which has tremendous and ongoing influence), and more.
I wrote a Twitter thread (was a bit irritated when I banged this out, unfortunately) that makes some similar points here with a couple of specific citations, newspaper clippings, etc. https://twitter.com/mbateman/status/1499590385638821889
I must admit that before my children started attending Montessori schools, I had this naïve view that Montessori education was somewhat similar to Waldorf, while on matters like this, they are pretty much diametrically opposed. It's somewhat amusing that there are multiple alternative school systems that claim for themselves to be "child centric" but have such startingly different theories of what children really want or need.