Just because, it's too much. I only see it working if Hulk is part of the story, and is playing "full asgardian" in terms of the plot, and relevance to the story. With the Hulk rumor, these new character introductions the obvious use of Thanos (since this is the last film prior to infinity war), and the fact that the norns talked about the infinity stones leads me to think that Thanos is taking Surtur's place and that Ragnarok is simply the start of the infinity war.
In fact, Age of Ultron was the start of something bigger. What if the cycle of "Ragnarok" starts with the Age of Ultron, and ends with Infinity War?
Dunno man, the only reason I see use of Surtur is because "Ragnarok". It already is packed enough. Loki and Thanos will be baddies in it, what would Surtur's role be?