[zintinio5]

I think your last point hits the nail on the head. There's a large amount of risk (especially at Twitter's size), and nobody wants to be saddled with an enormous and embarrassing failure. In addition, you need many hands to build the system, and finding developers for <favorite lang> that ALSO have relevant experience could be difficult. You could make arguments that developers in <favorite lang> are 10x as productive as your typical Java dev, but few people are going to take that chance. I'd say even Scala is still considered bleeding-edge at a lot of companies.