How could they? It's central to the entire core story. The Iron Born story can certainly be a little tiresome at times in the book, but there's some really interesting bits/scenes they could have farmed out of it, kind of a shame but I think they'd need 50 episodes a season if they went down that route.
Just when the Bran story was getting interesting. Maybe it's to account for the actor barrelling into puberty (I remember they did a similar thing in Lost). Having said that I haven't opened one of the books so I don't know if the Bran story goes away for a bit there too.
Meh I don't see what all the fuss is about the quality. These low res versions seem perfectly watchable for me. Tbh I don't even usually notice the difference once I start watching something. I guess if you're watching it on your big TV its more obvious maybe?