Sometimes it feels like trying to explain basketball to children. Let's make it a picture book.
First, here is the cut. You can see Rondo checking for the cutter. AD is already moving and Plumblee is about half a second from reaching Bron.
A fraction of a second later, Rondo has given up on the cutter and is looking at Bron. But Plumblee has already decided to crash down behind Bron. Rondo can't pass through Jokic's body to get to Bron (nor should he pass to the double-covered guy rather than the open guy), and Bron can't cut down because Grant is fronting him and Plumblee is coming to double from behind. LeBron could jump out to the 3pt line to receive the pass, but that would put him right where AD is going except with worse body position and bringing two defenders with him. Why would he do that?
Half a step later and Rondo has committed to AD. Imagine how difficult it would have been to get that ball to Bron considering where Jokic is. Jokic is huge. The only ways Bron gets the pass is by either jumping out to the 3pt line (which is exactly where AD is supposed to be) or by cutting inside. With Grant fronting him and Plumblee doubling from behind, he has no path to cut inside and get a pass from Rondo there. Where do you want him to "get open" to? Perhaps Rondo could have lobbed it over Jokic for a jump-ball situation, but what good would that do Bron? Even if he caught it, he'd have Grant, Plumblee, AND Jokic getting to him the same time the ball did.
Here's where AD catches it. Now, imagine that Bron had caught that instead. He's be triple-covered with literally no where to go, Grant right on his body with Plumblee and Jokic boxing him in on either side. Would have been a fukking wall.
Here's another angle - imagine where those three defenders would be if the ball had gone to Bron instead:
If Plumblee had chased AD, then LeBron could have spun off Grant and cut down. But with Plumblee where he was, where did you expect Bron to go to gather the pass, and how would that have helped the Lakers?
The way the play was called and considering where the defenders went, the Lakers got literally the only open shot available. There was simply no move LeBron makes there where he gets an open shot.