I just assumed since Left 4 Dead was only on Xbox 360 when it came to home consoles.
It was a different time then. A lot has changed. Since then we had Potal 2 on both consoles, and a closer relationship between Sony and Valve rather than MS and Valve during that time.
Where Valve's general attitudes lie now is anyone's guess, but given MS's W10 strategy (Or future strategy, however it shakes out ultimately), Valve and MS are becoming more and more competitors. Even the MS deal with Oculus seems to draw battle lines to a certain extent.
Unless things change pretty drastically between the scenes, I think it's safe to safe MS isn't going to be getting any kind of deal for the next Left 4 Dead.