Q - Why didn't the disciples understand that Jesus would die and raise again? even after he had told them several times that he would?
Short answer: The disciples have difficulty understanding because they have not been given the Holy Spirit yet.
Acts 1v3 reveals that they needed lots of proof. And v6 shows that they still have some old thinking. Then they receive the Holy Spirit at Pentecost and in chapter 2 Peter gives a sermon to the bewildered crowd that sounds like he gets it. And in chatper 3 he gives another sermon and explains again what God's plan was and is through Jesus dying and rising again. Look at verse 18 and 21 specifically. I'd say the disciples get it now.
Longer Answer -
I can't think of a specific verse that speaks directly to answering this question, saying "the disciples didn't get it because...". But I can give some verses that come to mind that might help put a realistic possibility together for you.
My first thought is that what Jesus says goes against our natural tendencies as humans.
-We tend to default to the natural over the supernatural.
-We tend to default to the plans or ideas that are most agreeable to us.
Jesus saying that He is going to die is not very agreeable to them im sure and people rising from the dead is not the norm.
So I could see even after they have seen Jesus die they might not think "Oh yeah Jesus said this was part of the plan ! Sit tight guys it's gonna be alright" but rather, they would probably think "what!!? Why is this happening!?? This is not how it's supposed to end!!"
Also, even though they had seen Jesus raise people from the dead, I could see that they might go back to their thinking -that once a person is dead that's it, it's over, and they don't get back up.
Why do I think this is a reasonable explanation? Well, because I know that I'm a fickle and forgetful person who most of the time doesn't grasp what God says with much strength of conviction and slip back into my tendencies like they are my favourite pair of jeans.
Take a look in the first part of Acts. In chapter 1 we have an example of how "the disciples didn't get it".
Verse 3 says - "After his suffering he presented himself to them and gave many convincing proofs that he was alive" . So Jesus thought that they needed extra proofs on top of the proof of himself standing in front of them.
Then immediately after this, we have another example of the disciples "not getting it" - he tells them not to leave Jerusalem so that they can receive the Holy Spirit and they ask in v6 "are you at this time going to restore the kingdom to Israel?". I'm pretty sure that they are thinking earthly kingdom and not the plan that God actually has because Jesus dismisses their question with "that's not for you to know", "but (here's what the real kingdom is about) you will go and be my witnesses all over the world".
Where I believe things change and the disciples start understanding is at Pentecost.
Take a look at the sermons Peter gives in chapter 2 and 3.
In Chapter 2 the Holy Spirit shows up and gives power to the disciples to be able to speak the gospel in languages they had not previously known. Many are amazed by this (rightly so), but some make up the ridiculous accusation that the disciples are just drunk. Often when people make an argument against God or what he is doing they abandon clear reasoning to do it. So Peter explains. And in his explanation he walks through God's plan of salvation, explaining it with details that were totally unique and unpredictable and profound compared to any ideas we could have come up with. For example - he mentions that it was God's plan all along and yet that they (or we) had killed him. Now how does that work? How is it our doing and yet not outside of God's sovereign plan? Somehow he orchestrates His will through us without compromising our free will? Peter also explains, that a passage where God speaks to David is actually not about David but about Jesus! And explains God's sovereign plan of sending His own son for our salvation and to be the new king in David's family line.
He does all this with almost a surprising clarity compared to the way he and the other disciples sounded so confused and off track with their questions and statements when we watched them following Jesus around. I believe it's largely because of the Holy Spirit acting as teacher and counselor.
Take a look for yourself at Peter's sermon in the next chapter and look at how clearly he understands now. Look at the details and watch as the disciples act with rock solid confidence on what they have come to understand.