While I am sure there are books/comics/whatnot that explains that, but that’s not explained in the movies. I admit my viewpoint is more on the strictly with the movies and the audience watching the saga from start to finish without having to go to outside sources to explain things, especially important/major aspects that affect the movies. (That is why I’m also personally not a fan of the sequels since they did not naturally flow from ROTJ like ROTS did to ANH, but this is neither here nor there)

So on my view (Not saying one is right or wrong at all!) it would be wise to keep things pretty self contained to the movies as I could say pretty confidently that the majority of people who watch the movies … just watch the movies. So keeping things just connected to the movies makes more sense to the audience. In ROTS, we have Palps talk about Plaguis and trying to stop death, or at least was “successful” in a way, but not fully (Plus palps did say he’s looking to figure it out in that movie too). So now, we see a connection to that WITH cloning, which was a big deal in the PT. Which may be the reason Palps was into a cloning for not just an army, but to “finish” plagues’ work.

So just in my modest view, having things stay more connected in the movies, even if it is simple explanation or connection, it feels more connected that way. Because in some ways, having to “retcon” or “rewrite” aspects of the previous two sagas feels a bit cheap … or at least “forcing” what happened previously to accommodate the new.

So again, not saying there is “one right way,” but just a view to help keep the movies connected together.