I've consulted for them and experienced this first hand.
I think the short and perfunctory season of GOT was due to Time Warner wanting this done quickly. They wanted it done for accounting purposes to prepare for the merger.
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That was all the showrunners. HBO asked for 10 seasons in total and offered a blank check more or less, but without good source material after season 5, Benioff and Weiss just weren't able to create the same kind of character development as GRRM, and they knew it, so they would only agree to 8 seasons, with 7 and 8 being short seasons, and that was way before the AT&T acquisition. GoT was totally character driven, so, without that, it kind of doesn't work as well.