It's the same reason Napster was so valuable before the RIAA shut it down and destroyed it.  

Napster had captured the entire online music activity under one roof. A virtual monopoly. From there, in hindsight, you could have launched what eventually became iTunes, and had immediate huge market penetration from Day One. Apple would have made devices that hooked into Napster. They wouldn't have controlled the revenue flow of the music industry as they effectively do now. 

Same with Twitter. The whole of the microblogging world is under their roof. It's the prototype for the news system of the future. Under competent management with a longer-term view and deep experience with news, Twitter would sweep the whole news landscape into its domain. To have almost exclusive control of the flow of news for a large portion of the world -- could that be worth $10 billion? Obviously, that and a lot more. $10 billion would be a bargain. 

The Wall Street Journal employs writers and editors who were schooled in a world where the possibility of wrapping up the whole news platform was unthinkable. That's why they have so much trouble thinking about Twitter. 

Hat tip to Felix Salmon for seeing this too, from his own point of view.