Why no Shivan Dragon?
Short answer: no room
Longer answer: You could replace the inferno titan or a couple other spells with the shivan. For me based on what I wanted in the deck it was a choice between the shivan or the siegebreaker. With all the token and aggro decks running around the 2 damage to all non-fliers on attack I thought was better than increasing combat damage for spare mana. While not a dragon the inferno titan fills a similar role to the Shivan and has the added benefit of picking off small creatures.
Hopes this answers your question