Because it's hard to fit a Great Wyrm on stage, yo.
Seriously, I can't even find the words to describe how much I love this. Smaug was my favorite part of The Hobbit (the book), as well as The Desolation of Smaug. And the fact that uber-Tolkien-nerd Stephen Colbert was able to arrange this interaction is nothing short of amazing. I feel like all of my molecules are going to explode into micro-squees.
And here's the Comedy Central video: