Lets not forget the C-17s were in TA only and weren't taking part in the resolution advisory calculations. From what I've read, the AA plane actually had a reversal of commands; first a "descend now", then an "increase descent", and finally a "climb now". There was actually ZERO vertical separation when it was all said and done. Why that happened, no one really knows yet.
It was night. I sure as hell can't tell whether or not someone is at my altitude or on a converging course at night. And how do we know they weren't in a cloud layer.
OH lordy! The 777 was on it's own! The C-17s were only in TA! DOOOM! Are you guys listening to yourselves? Seriously -- to listen to some of the points of view I read and hear, it is amazing that aircraft were ever able to fly without having a midair before TCAS was invented. Who cares if they're on different radio freqs -- my radio has never been able to look out the window and alert me to traffic. Night ops? Yes, this was over the United States, and they had these big red things flashing on them, along with a couple other colors going, too. Again, this is why there are windows and people in that room at the front of the jet to look through them.And yes...the TCAS worked. Mostly. The TCAS RA on the 777 commanded a descent, then an increased rate of descent, then a reversal to a climb. THUG11, the flight of two, had their transponders set to TA only and couldn't receive RAs; there was no "negotiation" between the systems of each aircraft. The 777 was on its own for avoiding the C17s.
It's not such a small point that the RA commands provided confusing and contradictory guidance. You ask "why that happened", and I think that's a very easy answer -- because TCAS is a very imprecise tool that reacts comparatively slowly (compared to what a pilot would do under similar circumstances), especially if the other aircraft is maneuvering (or if the TCAS containers aren't talking to one another, of course). This is EXACTLY why pilots need to remember that TCAS is an aid to clearing. Say that again -- TCAS only supplements the pilot's responsibility to look outside and miss stuff. Yes, even at night. I have flown with a number of USAF pilots (from non-tactical backgrounds) who in practice rely completely on the hits on the display to cue their visual search, and that's only IF they are actually conducting a visual search to begin with. More often, it's the "Traffic" call that makes them look up from their magazine or stop fiddling with the FMS.
Don't get me wrong -- I like what TCAS does. It's a great backup. It has alerted me to stuff I did not see a number of times -- I'm not going to claim that I have perfect visual lookout (and my old man eyes are juuuust starting to fail anyway!). It's nice when you're in the soup and it gives you SA on what's going on around you that you would not otherwise have. Just like every other piece of avionics, though, I assume that it's trying to kill me and inherently don't trust it. It is only a part of my SA building.
Flying in a big aircraft with a multi-pilot crew and all sorts of fancy toys doesn't ever negate what your primary responsibility is as an aviator, nor abrogate the skills and habit patters that you were using to fly 150s around to get your PPL.