Quoted from Ranhorton:

So I don't have a dog in this fight (never seen the show GOT and don't have interest in the game) but I can speak from experience with regards to the art and licensing.
I work in the creative department for an apparel brand that does TONs of licensing with all the biggies and we need to get everything approved by the license holder. We were just working on some new Star Wars The Force Awakens stuff and all of it was shot down by Lucas even though we used the approved artwork they sent us. The problem was we took minor creative liberties with it to better suit our product needs. To them it was just enough to reject and send us back to the drawing board - and force us to create something twice as fast with less creative expression.
I guarantee this happened to Stern while working on GOT.
I don't blame them at all for the artwork situation.

Quoted from rai:

While Drogo was an main character for a time, he's not on the level of Ned Stark.
First Ned is the first main character in the books, introduces the series and you'd have thought the whole series was going to be about Ned. But past that all Ned's children as well as the parts Ned played in the back story about the rebellion against the Dragon lords. John Snow and all the children have major parts.
Wereas Drogo did not have any surviving children and is more like a colorful interlude to the entire story and has nothing at all to do with Westeros.