Quote from e-freakEarlier Teams (or at least theyr leads) came to Seattle, Washington and worked at Valve's HQ but with Left 4 Dead's Developers they made a step which other Companies did a long time ago: They start a studio in another place ...
Earlyer teams were modders and students who didn't have a office/company as far as I know.
So maybe the Turtle Rock guys (ex westwood right?) just didn't want to move.
I find it a bit weird for a big studio like Valve (100-150 people last numbers I saw) and their loose policy of "work on what you want when you want" to start a satelite team of what, 10 people?