People are horribly stupid when it comes to casting. Even former actors who should know better can't stop putting good actors in boxes. My wife, also an excellent actor, has moved into casting and is rapidly building a reputation at being great at it. The people she finds regularly bring more to their roles than even the role's authors knew was there. But she has to fight people all the time on whether actors fit an overly specific look or preconceptions they have about what their ethnicity or race will "bring" to a role to the point of trading off spectacular amounts of talent for mediocrity. This is in spite of what anybody who watches theater regularly should know. Actor talent or lack thereof can make or break just about anything. Actor stereo-typing is very simple-minded but also very common and everybody knows Star Wars. Directors and casting directors should know better but being in any popular sci-fi/fantasy film can be a tough box to climb out of.
That said, Portman hasn't been more than good enough for a role since she was a kid and I haven't seen Christensen in anything else, but the dialog in SW has always been a challenge and I don't 100% blame directing for them looking as bad as they do up there. Those actors in the original trilogy are working VERY hard in almost every scene. Neeson and McGregor rarely looked bad but you better believe some of that dialog reads something awful on the page. Only the empire deals in absolutes? He made it one more sentence beyond that before it occurred to me what a stupid line that was. That takes talent. Not to mention his gradual morphing into somebody who might actually be Alec Guinness in 20 years. Impressive work.
But let's completely black out Ep 2's romantical dung-heap. It's what I do most of the time. Even if we ignore that dialog/directing horror, Christensen pretty much manages to hit two notes through the entirety of the last two prequels, surly and remorseful. I'm sure directing didn't help him there and the role wasn't written with much opportunity to play for charisma but that's something the right actors can excrete through their pores if they have to. Ford's done it his entire career. He could have played being mad about his lack of rapid-Jedi-career advancement like it was a legitimate gripe. I mean really, this is a dude that's done some pretty amazing things for the Jedi and in return he gets way too much free time to hang out with evil senators and dead mom. But instead he comes off as a self-entitled brat. The right actor could have made me like him at least a little bit more but in the end the balance was tipped in favor of rooting for the lava.
Portman, even when given interesting things to do like be an action heroine, the ruler of an entire planet, or its representative in the senate is 99% flat-lining throughout the whole thing until the very end when she's heart-broken at which point she demonstrates her vulnerable pretty girl thing that she's mostly been hired for. Unfortunately it's far too late for anybody to care that her character is heart-broken. She's just not that good. She can do drama on the extremes and her narrow shtick but she's not up to being a mover and shaker in a galactic space opera that makes no sense whatsoever if you think too hard about it.