My fan theory
I'm back. So I still haven't got any idea on why the ghost crew or anyone really would abandon Rey in the first place. I did think about another possible connection between Rey and Ezra. What if they actually have the same home world. I know Ezra's from Lothal and Rey's from Jakku but just hear me out.
Ok so Lothal is supposed to be this beautiful grassy plains that is broken up by hills and rivers. Rebels stays pretty true to this except that it never shows the rivers or any water really. The episode Visions of Hope in session 1 has a mural of a fishing industry on the planet and Ezra says that the sewers used to be to full to travel through but that since the empire started rationing it's pretty dried up down there. My question is why would you ration water on a planet that is known for a fishing and farming industry. There should be lots of water there. I've also noticed that in the Servants of the Empire books they talk about rivers and streams. They visit a farm with a stream running through it. The empire starts mining the farm and the stream becomes polluted & dries. So as the empire strips the planet of it's resources the planet dries up. You following me here?
Good, now note Lothal is also a vacation planet and who would want to go to a planet that has lost it's famous attractions? No one. So they lose their farming and fishing industry and then they lose their tourist industry. The planet becomes a desert planet. By this time the rebellion has actually won the civil war and the empire has fallen. The First Order is in its early stages. Ezra is still alive. Leia knows that his home world is slowly being destroyed. She knows what it's like to lose your home world and the two decade to try and gain control of the planet and save it. However once they get there it's too late. Lothal is no longer the planet it used to be. It's almost a different planet that no one wants to visit or live on willingly. The Republic renames the planet Jakku. They have to fight to keep control of it. Eventually it is abandoned by both the new republic and the first order.
Once there becomes a threat to the future of the new Jedi order Ezra takes his daughter to his home world which is now known as Jakku to keep her safe. He leaves her in care of a friend of the ghost crew. He left her a note saying he'd come back for her when it's safe for her. However it only gets worse.
Ezra never gets the chance to go back for here. With the attack on the new Jedi order and Luke's disappearance Ezra realizes it will never be safe for him to get his daughter back. Him and Sabine both know that they may never see their daughter again. Once Ben Solo falls to the dark side and Leia & Han's marriage pretty much ends all hope is lost for the Ghost crew.
The team then separates. Sabine becomes a famous artist in the republic. She is also an arms dealer. She supplies and supports the resistance. Ezra goes into hiding as well and starts broadcasting mysterious messages. Everyone else thinks he's dead. Only Luke and The ghost crew knows he's the one broadcast the messages or even alive. Kanan Chopper and Hera run a shipping company together. Zeb becomes a representative of his species. This is assuming that they are all still alive. It's actually possible that some of them might be dead by that point.
Anyways so when Rey shows up again no one has seen her in a long time. Han and Leah did both know her but it had been so long since they had seen her that nether recognize her at first. Kylo Ren does because as a child he knew her better before she was put into hiding. Ben actually cared for her and looked out for her. Luke was one of the last people to see her or Ezra. That is why part of the map is on Jakku. He gave it to Ezra to send to her with a letter explaining why he wasn't coming back for her. She never got it but some else found it just before the Force Awakens begins. Rey is supposed to leave Jakku and look for Luke and the Ghost crew. Ezra asked her to reunite the Ghost Crew as well as the crew of the Falcon or at least what's left of them anyways. She just doesn't know this. Maz knew the crew of both ships and knows what Luke and Ezra wanted. She Knew that one day she would show up at her cantina. What she didn't know was that Rey never got Ezra's letter and was still expecting her family to return for her.
After Han dies and Rey comes back with Anakin's (I'm calling it that because he built it. Luke made his own. Rey will too.) lightsaber Leia finally realizes who Rey is. She knows what Rey just found out and goes to comfort her despite her own grief over Han dying. She puts that aside to do what she knows has to be done first. Leia does the same thing in A New Hope. She's not ignoring it but addressing more important issues first. Rey was there when Han died and she had just learned about the fate of her family and the force. That's a lot to take in at once. At least Luke and Ezra got this stuff a little slower. Ok maybe not so much for Luke but he was never abandoned. Ezra and Rey were and both around the age of seven at that. Leia knows that Rey is going to need a teacher if she is going to complete her journey. She'll help her anyway she can. She send Rey to get Luke for that reason while she gets however she can from the Ghost crew. Sabine will probably come running as soon as she hears about her daughter. By episode 9 what's left of both crews will have resurfaced. Rey's first mission will be complete as will her training. This will be the end of the Skywalker's story but the Bridger's story is still going on.
Disney is creating a new family to follow the story of because there is nothing left of the Skywalker story to tell after this trilogy. The torch has been passed from one family to another and Excalibur has been handed down. Luke has passed along what he's learned.