Granted, I did not watch a lot of games all the way through, so I can't speak to the poor fourth quarter play -- but assuming it's correct ...
Usually late-game struggles are more mental than physical. Again, games aren't that long, and these are elite 20-something athletes in very good condition.
(Digression: When I was a young coach, I said what pretty much every young coach says: "We will be the best-conditioned team on the floor" -- and I ran kids to death. Now I do almost no conditioning, letting the drills do the work. Very few top teams at any level grind really hard because the game is played or worked on year round, and when you have athletes pushing themselves in the weight room, injuries are all too common.)
Part of it, I think, is the nature of the game. For the first three quarters, say, you can implement your strategy for the game, your new sets or whatever, but by the end of the game the other team knows what you want to do and has seen it a bunch of times. So have to revert, in some ways, to just playing basketball -- and here's where familiarity and continuity come into play. If you've played with someone for a while, and the other team is taking away your plans, you become more reliant on knowing what your teammates like to do, and what you like to do.
Also, it's a reflection of an overlooked skill: The ability to focus for an entire game. Some people have the mental ability to stay on point for longer than others, but most people have trouble keeping full concentration under pressure for long periods of time.
You can practice that and work on it, but it's hard if kids are going to class and dealing with academics because they are mentally exhausted when they arrive at practice. And again, I think continuity makes a difference, because if you've been at a school for two or three years you understand the overall situatiuon and you're less likely to be surprised and stressed -- which impacts your ability to concentrate.
So you have two competing factors: Familiarity on the court with your top seven and mental fatigue after a demanding game. You can play more kids but you lose familiarity, or you can stick with your top players and hope they can stay on point.
Coaching isn't easy ...