If you look closely at the video, you see the lead official--working along the baseline under the basket--immediately signal that the block was clean and they should play on. I can't quite tell if he was looking at the ball and the three sets of hands around it at the time of the goaltend, or if he was looking low for body contact between the shooter and the defender.
You can't see the other two officials in the video of the play, but the offside ref--working just below the free-throw line on the near side of the court--should have also had an unobstructed view of the basket to see the hand come up through the net, inside the rim, to get the block. The trail official--working on the ball side from beyond the three-point line should have been able to see the rim in front of the Gonzaga player's wrist--but again, I have no idea where he may have been looking.
Like the professional leagues, the NCAA uses "All-Star" crews for the basketball tournaments. Officials who grade out the highest in the regular season are brought together with partners they may not have ever worked with before. Perhaps this crew didn't communicate to each other in their pre-game meetings who would be responsible for watching what on plays around the rim. Or perhaps each thought the other had a better view and nobody ended up making a call. Or somehow, all three guys never saw the hand--and the net--go above the rim to illegally knock the ball away.
The obvious next questions is "Why didn't they go to replay to see if the hand was inside the basket?" The simple answer is that such a play is not reviewable under college basketball rules. We can stop the clock 15-times in the last minute to review every ball that is tipped out of bounds or to see if we should add two-tenths of a second back onto the clock because the timer didn't hear the whistle on a foul--but to overturn an obviously-blown call that could affect the outcome of a game is off-limits.
Then, the situation gets compounded by Northwestern Head Coach Chris Collins storming onto the court demanding a goaltending call--which necessitated a technical foul call--killing all momentum for the Wildcats. While a number of "experts" said that Collins should have kept his cool and this his "T" actually cost his team the game, I give him a ton of credit. If I was coaching Northwestern, I would have received two technical and been ejected immediately--but not before I threw half the chairs from my bench on the court--along with my suit jacket, tie, both shoes and the video replay equipment from the scorer's table.
Of course, the NCAA issues a statement the next morning that the call was clearly blown--but that Collins was wrong to protest in the way that he did. Needless to say, the three "all-stars" working that game won't be on the floor for anymore tournament action this year.