It's not just because of the art. That's one piece of it.
It's also because contrary to popular belief, Kingdom Come was not a universally beloved story and it's a story now that Mark Waid has alot of regrets about, in part, because of the presentation of the women in the book.
Kingdom Come is a popular book but it's also a book with severe, severe gender problems that have often been pushed under the rug because the art is so beautiful. But they weren't problems that went un-noticed.
Wonder Woman's portrayal in KC is intensely problematic. Mark Waid has admitted in recent years that he really knew nothing about Wonder Woman and that he has deep regrets for the way he wrote her. The way Lois Lane was fridged in the original story (off-page, treated as nothing more than a corpse/plot device as opposed to given any personal agency) is also really problematic and has been the subject of a ton of criticism. The way she was carelessly fridged also led to copy-cats for years on end wherein writers killed Lois off in violent and sexist ways to create angst for Superman. KC created alot of problems in comics for women in many respects that are still repeated today over and over again and was, in some ways, responsible for this obsession with a dark, gritty Superman. The truth is that Mark Waid wrote Kingdom Come as a "WARNING" about what "not to do." He didn't write it as a "how to" guide. Yet, so many people misunderstood the message of the book and assumed that this was supposed to be a story of how things were supposed to be. And that was NEVER the point. The point of the book was to be a warning about what not to do.
I understand that some people like the book but there are also alot of people (yes, even professionals) that DO NOT like it.
Personally, for me, the gender problems with Kingdom Come are not outweighed by the pretty artwork. I don't like the book. I don't like the way women are treated in the book. I have no desire to ever see the book made into an animated fillm and I'm glad that more and more people now are waking up to the problems of the book and really looking closer at some of the damaging messages that the book sent. ::shrugs::