A few reasons - almost entirely based on meeting him,
I met him at C2E2, but never planned on buying a copy of the book there as I already had 2 ordered. He didn't even seem like he wanted to be there, and this was early on Friday.
At least when I was there, his booth consisted of a lone copy of the Fett variant on a stand, no price displayed, and 2 "handlers" behind his booth only to say "$60… It's $80 on ebay". It was the weirdest creator meet I've ever had. He seemed pleasant enough if I wasn't buying something from him - but some people you just get an insincere vibe that rubs you the wrong way… that's what I got and it's not something that's super easy to describe I suppose.
Semi-related weird side note: his signature (which it's his right to sign however he damn well pleases) looks literally like this: ~~----------. Zero exaggeration there.
Had I not met him in person, I probably wouldn't have as much bias… but I have yet to hear from someone who met him that felt he was genuine.