Didn't even consider that, probably the most likely answer. Doesn't Valve (for good reason) hate Microsoft after the crap policies Microsoft had towards Valve regarding Left 4 Dead updated and charging for DLC?
Just taking a guess since L4D1 and L4D2 were console exclusive to Microsoft as well. :| Asking a question since the first leak of this came from a Microsoft employee (you), presented to you in the form of a PowerPoint (seems like it), so the possibility is there.
Plot twist: The whole house is now destructible. Your party is on the front porch when the tank spawns, he takes a swing. It misses by a fraction of an inch but strikes the pillar holding up part of the front of the house and the floors above it. The debris falls and kills your entire party. Cue left 4 dead 3 failure tune.
It was exclusive only because Valve couldn't be damned to port it to the PS3. Orange Box was EA published and ported by another team I think, either that or valve gave up after the performance issues on ps3.