1. after 15 years, incest is really not an issue with the viewer base of GoTThe Porkchop Express said:
The problems with telling Aegons Conquest are the nonstop incest and that every matter is eventually resolved by burning the enemy with dragon fire.
I would love to see Robert's rebellion or even a bigger show with the dragons getting tiny and then going extinct, the defense of duskendale, the prime of Arthur Dayne, all that stuff.
2. Still plenty of skirmishes and political intrigue that can be covered, some of which they lose. And the storyline can cover from 1 year before AC to years 6 or 7 after the Dornishe wars and end with the Dragon's Peace.
3. With HBO is such a penny-pinching mode, no way they are going to devote money developing D- tier storylines or characters. Yes, Dunk/Egg are C or D tier, but there is a lot of established story around them on which to builds a series, so could be seen as a less risky production.