Yeah, I read they avoided offers for a live action for years but I'm not sure why it's sacrosanct. Maybe it came in "early" qua graphic novels (versus comic books).
In "An evening with Kevin Smith" (which you can buy, or find on Youtube), he details how around the same time Jon Peters was trying to persuade him to make "Superman Returns" which featured a "Giant Mechanical Spider". Neil Gaiman also got this pitch for a Sandman plot. Eventually the giant spider ended up in "Wild Wild West" with Will Smith. We all know how that went.
Well, it's a pretty good fantasy comic for adults. It's not the only one, nor the first, but it's pretty good. And fans tend to be zealous of those. For many of us, it holds a special place in our comics-reading education, and so we would not wish to see it demeaned but yet another low quality adaptation that completely misses the point or dumbs everything down.
Thankfully, it doesn't seem to be the case with this adaptation of The Sandman :)
edit: this is what Gaiman has to say about The Sandman's cultural significance:
"And after 30 years, “Sandman,” at this point, is probably the single best-selling series of graphic novels ever published in the U.S."
