The Roy Williams trade.
That single decision is probobly the reason why a team with this many stars failed to reach the superbowl. It's extremely rare that you have a top-tier QB, a top-tier pass rusher, a top-tier left tackle, a top-tier running back, and a top-tier wide receiver and not sniff championship games.
But the Roy Williams trade set everything off. The trade lost valuable draft picks that could be o-linemen or safeties, it touched off the desperation trade-downs and draft picks such as Robert Brewster that resulted in zero players remaining on the team after 4 years, and it chewed up 50+ million that could have been used to sign o-linemen, safeties, etc.
That single decision of basically throwing so many draft picks and so much money in the garbage, tying your hand behind your back, meant that the team couldn't add the players to its roster that they needed to. Think about it, if the league was to penalize a team with a $50 million salary cap penalty and the reduction of a 1st, 3rd, and 5th round pick it would by far and away be the biggest penalty in NFL history. Such a penalty would be viewed as being insanely punitive and people would protest the league offices for such a crazy penalty, they would say a team suffering this penalty could never recover because it would be so onerous... and yet that's what the Cowboys did to themselves with the Roy Williams trade.
All this team needed were a couple average role players in spots like the o-line and safety to shore up weaknesses and put it over the top but those role players never came on the roster because the draft picks and dollars that would have been exchanged for them sit in Roy Williams' and the Detroit Lions' bank accounts.