(this is all idle speculation and I have no insider info)
Second season is coming in 2020, so could it be that Uni might have just figured that waiting until the second season announcement is on everyone's minds might be a smarter move? Plus, this was the year we were supposed to get a GO(A)T house, so it's possible they just spent all their negotiating power on that and chose not to pursue Hill House, which is made by a company separate from Netflix (Paramount Television and Amblin) and probably involved a lot of rights headaches that people just didn't want to deal with along with HBO.
EDIT:
@DavieDarko I read somewhere that Rob Zombie's Halloween II has some of the most realistic depictions of PTSD in a horror film - with all the stuff about Laurie hating Annie because she was a trigger for her trauma, her drinking and engaging in self-destructive behavior after every mental breakdown, her anger at everything, her fear of being committed to a hospital, her listening to edgy music to try and distance herself from her past self - being completely true to life. Everyone thought she was overacting, but I thought the actress put her all into that performance. That same commentator said H20 got it completely wrong, funnily enough.