The Last Jedi answered some questions and raised a great many more. Luckily, Disney and Lucasfilm aren't going to make us wait until Star Wars 9 to reveal everything we want to know. Or, much worse, not tell us at all. One mystery that The Last Jedi didn't answer was why the Rebels abandoned the planet Crait, which is where the climax of the movie takes place, years before the events of this new trilogy. Now, thanks to a new Star Wars comic book, we have our answer.
Warning: spoilers ahead for Star Wars: The Last Jedi - Storms of Crait #1. The new one-shot comic from Marvel takes place during the events of the original trilogy. Roughly speaking, not long after the events of Star Wars: A New Hope. To understand why the Rebels had to abandon Crait, a little backstory is necessary. The planet first appeared in the Star Wars canon in the novel Leia: Princess of Alderaan. In the book, it's revealed that Princess Leia's father, Bail Organa, had set up a Rebel base on the planet Crait. That's important because in Storms of Crait, it's revealed that the planet at the time the Rebels abandoned it is run by Trusk Berinato, an old acquaintance of Bail Organa.
In the book, Han, Luke, Leia and Wedge Antilles head to Crait to meet with Trask Berinato. He actually turned the planet into a mining facility, which is why there are all of those caverns for the Falcon to fly through in The Last Jedi. We discover that, following Bail Organa's death on Alderaan, Berinato turned to the Empire and began working with them. Instead of just bailing out, Leia and the gang head to Crait to try and work things out with him. Leia says this during a confrontation before things get ugly.
"Mon Mothma doesn't trust you. But I want to. We know that you have a relationship with the Empire, Trusk."
Unable to come to an agreement, a fight breaks out and thus, the Rebels must flee Crait for what they likely think is for good. It isn't until years later that, during the final battle in Star Wars: The Last Jedi, the abandoned Rebellion base actually manages to save the few forces holding together the shattered Resistance.
Not knowing this information doesn't hamper the experience of watching Star Wars: The Last Jedi, but Disney and Lucasfilm do a great job of making sure that fans who want to know this stuff can find out in an entertaining way. This also, though not said explicitly, explains why the Rebels have their base on Hoth at the beginning of The Empire Strikes Back. If you want to read the full comic, Star Wars: The Last Jedi - Storms of Crait #1 is available now from Marvel Comics.