I think that is mostly because it's just one of the episodes, story-wise. Splitting the game wasn't really a good idea for the flow, no matter how much a variety the missions provided.
Good point...i'm probably also being a tad biased as I was very dedicated to starcraft's storyline.
I spent 5 years drawing out fan art and writing fan fiction for the starcraft universe. I was so enveloped in it, that when it came to blizzards actual rendition of where the story left off, i was completely disappointed.
I used to draw Diablo comics too...but they were more humorous and left very little room for storytelling and character development. I did enjoy drawing out sweet armor though