HBO can't make everything, damn it.
It's fascinating to me how "Make it a series for HBO!" has become a rallying battle cry for genre nerds everywhere any time there's something that seems hard to figure out as a feature film. And while I'm sure HBO would happily put every single thing ever on the air, that's just not economically possible. They have to choose, and sometimes they don't particularly love having to make those choices because they end up losing material that they would like to make simply because they don't have enough room or time to produce every show.
Michael Lombardo recently expressed excitement about the possibility of doing a "MaddAddam" series with Darren Aronofsky, and in the same interview, he talked about how upsetting it was to lose the rights to "American Gods," which they tried to develop for a while.