the showrunners did mention in an interview that they got HBO to add an additional 4-5 minutes to every episode, which basically means they get an 11th episode in terms of total minutes for the season, so there is that. And as previously mentioned, seasons 3 and 4 are book 3 of the book series so there's that too.
I think that they can't go for more than 10 episodes probably for budgetary reasons. The show costs a lot of money to make because of all of the different locations used for filming, plus with its huge cast that's a lot of actors to pay for. Also, there's more CGI on this show than on other HBO shows, so it costs a lot more money to make each episode. Although, HBO is making a lot of money off of this show's DVD sales so maybe in future seasons they'll add some more episodes, who knows?