Personally, I find his work compelling - I saw an amazing exhibit at the Frye in Seattle and the scale of it was incredible, also the composition. But I think his work makes some people uncomfortable. From Wikipedia: "One idiosyncratic feature of Darger's artwork is that his girl subjects are shown to have penises when unclothed or partially clothed. Darger biographer Jim Elledge speculates that this represents a reflection of Darger's own childhood issues with sexual identity and homosexuality."

This is the best (only) copy that I have been able to find that appears to be legitimate (?). It only includes selections, is of low quality, and is not the entire work -- but it does provide some legible pages:

https://archive.org/details/realms-of-the-unreal/Realms%20of...

This book also supposedly includes excerpts from his writings, but I didn't check: https://archive.org/details/henrydargerdisas0000darg/

From all accounts, it’s—not good. Also, it’s really long and it would be a non-trivial (and expensive) effort to make it available.

A lot of problems like the one you're describing boil down to commercial rather than technical reasons.