The Sequences are inherently short, there are just massively many of them - the fact that each one is woefully inadequate to its own aims is eclipsed by the size of the overall task.
The longer stuff, Siskind included, is precisely what you get from people with short attention spans who find it takes longer than that to justify the point that they want to make themselves. There’s no structure, no overarching thematic or compositional coherence to each piece, just the unfolding discovery that more points still need to be made. This makes it well-suited for limited readers who think their community’s style longform writing is special, but don’t trust it in authors who have worked on technique (literary technique is suspicious - splurging a first draft onto the internet marks the writer out as honest: rationalism is a 21st century romantic movement, not a scholastic one).
Besides which, the number of people who “read all of” any of these pieces is significantly lower than the number of people who did so.