It's not like he shopped it to every publisher around and shown the door.
Thompson said it got rejected at least seven times by various publishers. He wrote it when he was 22 and didn't publish it until 1998 after decades of clear writer's block due to his drinking problem.
I love Thompson: Fear and Loathing, his Hell's Angels book and parts of The Great Shark Hunt are terrific and he even did occasionally inspired short pieces into the 80s but the publishing of this juvenilia was the act of a self-admitted burn out.
CQ, I have read it and I think the Thompson who wrote Fear knew it wasn't up to snuff. But it isn't the first time a writer has had some inferior work published, look at when they put out Ellison's unfinished Juneteenth after his death. This marginal stuff is of interest to scholars and cult followers, but if someone picked up Rum Diary instead of Fear they'd wonder what the hell all the bother was about and that just hurts a writer in the long run.