That way you have a clean slate post ROTJ, while keeping The ST confined to it’s own timeline.
Have your cake, and eat it too.
Honestly, I think one could make the argument that all of Filoni’s series take place in a slightly different timeline from the movies. Even with the brief Hayden Christensen voice clips in TCW season 7, I still don’t buy that Anakin from The Clone Wars would fall for what Anakin falls for in Revenge of the Sith (I mean, he would still become Vader, but it would take a lot more convincing). But this isn’t the right thread for that.
Back to the latest episode of Mando, was it just me, or has Ludwig Gorranson not used any of John Williams’ original motifs aside from the Force Theme? When Luke fights the Dark Troopers, there’s clearly a recurring melody, but it isn’t anything I recognize - which is weird because the tune is in harmonic minor, the same scale as The Jedi Steps, another suspenseful piece used to build up that Luke Skywalker is a big deal. So why not incorporate that?