Warning: major spoilers ahead for the two-part Season 4 premiere of Star Wars Rebels, called "Heroes of Mandalore." If you haven't caught the episode yet, you may want to check out until you have the chance to watch.
Star Wars Rebels has officially returned to the small screen for its long-awaited premiere of the fourth and final season. The two-part premiere took the action to Sabine's home planet of Mandalore, as she and members of Clan Wren embarked on a desperate mission to try and rescue her father before he would be executed in the capital. Kanan, Ezra, and Chopper were on loan from the Rebellion to help Sabine, and they got an assist from Clan Kryze courtesy of Bo-Katan (Katee Sackhoff), who made her first appearance in Rebels after playing a memorable part in The Clone Wars. In the course of the group's desperate mission, "Heroes of Mandalore" might have finally given the answer to the question of why there was no Mandalorian presence in the original Star Wars trilogy.
Many Star Wars fans know that the Mandalorians have been a pretty big part of the franchise over the years. They had a large presence in the Expanded Universe before it was wiped from canon, and they were important to certain aspects of the much-loved Knights of the Old Republic game. Mandalorians also had a prominent role in The Clone Wars as a warrior race being ruled by a determinedly pacifistic leader by the name of Satine. Mandalorians have been established in the Expanded Universe and currently-accepted canon as some of the fiercest warriors of that galaxy far, far away, which led fans to wonder where they were during the original trilogy. The closest we got to on-screen Mandalorians in the trilogy was Boba Fett (who is technically a clone in current canon) in his Mandalorian armor. Now, we may know why they were nowhere to be seen as a fighting force in the original trilogy: they were busy on their homeworld.
In "Heroes of Mandalore," Sabine and Co. discovered that the weapon she designed back when she was a student at the Imperial Academy had been rebuilt by the Empire with devastating results. The weapon was able to hone in on the special properties of Mandalorian armor and take out anybody within a certain range who happens to be wearing Mandalorian gear, while leaving Stormtroopers untouched in their Imperial armor. A whole group of Mandlorians was wiped out by the weapon when it wasn't even at full power, with Sabine's mom and brother barely escaping with their lives. The surviving Mandalorians--including Bo-Katan--were initially furious at Sabine for creating the weapon, although Bo-Katan convinced everybody that Sabine was necessary to take out the weapon for good.
Well, the plan to destroy the weapon went sideways pretty quickly, as Tiber Saxon captured Sabine and Bo-Katan, threatening to kill Bo-Katan if Sabine didn't fix the weapon to increase its range and enable it to kill even more Mandalorians in their armor. Sabine seemed to agree and made some adjustments to the weapon... only to reveal that she tweaked the settings so that it targeted Stormtrooper armor instead of Mandalorian armor, therefore giving her the power to incapacitate and even kill throngs of Imperials wearing their armor.
If you're anything like me, the introduction of a weapon that can take down a ton of Stormtroopers from an impressive distance shortly before the events of the original trilogy seemed like a huge plot hole. After all, if Sabine had a functioning weapon that could take out masses of Stormtroopers, the Rebellion probably could have made much faster work of the Empire than the three movies of the trilogy. Luckily, "Heroes of Mandalore" quickly took the weapon out of the equation, removing any thermal exhaust port-sized holes in the plot.
Although Sabine was on board with using the weapon to take out the Imperials, Bo-Katan stepped in and convinced Sabine that using the weapon would mean defeating the Empire with their methods rather than Mandalorian customs. So, Sabine destroyed the weapon, the Mandalorian rebels earned the wrath of the entire Empire, and the surviving Mandalorians pledged their clans to Bo-Katan to lead in a battle against the Empire.
Basically, the Mandalorians now have a lot going on in their own battle against the Empire without any troops or resources to spare for the larger Rebellion. Even without the weapon that vaporized Mandalorian armor, the Empire has almost all the advantages in the war on Mandalore. Even if Sabine does return to the Rebellion, the majority of the Mandalorian warriors will fight for Mandalore on Mandalore, not out in the entire galaxy. Thus, no Mandalorian forces in the original trilogy.
We can only hope that Star Wars Rebels will explain Sabine's absence from the original trilogy by sending her back to Mandalore in the finale or giving her a behind-the-scenes mission. Given that she has had one of the most compelling arcs of the series, it would be a bummer if she died at the end. At least her odds of survival are better than Kanan's, although that's not saying much. The writing may be on the wall for poor Kanan as a Jedi in the days leading up to the beginning of Rogue One and A New Hope.