Because Jesus knew from the beginning of his decision not to believe. Jesus warned about falling away or turning back. Judas did.
Judas fell away, there is no disagreement about that. However where we differ in belief is that you believe he lost salvation, but I really don't believe he was saved at any point of time.
1 John 2:19 They went out from us, but they were
not of us; for
if they had been of us, they would no doubt have continued with us: but
they went out,
that they might be made manifest that they were not all of us.
I don't believe that Judas was given to Jesus.
How do you know any of them were given? Because they didn’t fall away? Is that how you know you are “given” because you haven’t yet fallen away? If you later fall away does that mean you were actually never given?
I know that they were given to Jesus because Jesus said so:
John 17:12 While I was with them in the world, I kept them in thy name:
those that thou gavest me I have kept, and none of them is lost, but the son of perdition; that the scripture might be fulfilled.
This doesn't mean that Judas lost salvation, or being "given and lost", it means that the son of perdition was put into that office, so the scripture may be fulfilled (by someone betraying Jesus).
If I am wrong, and Judas was indeed given to Jesus' hand but lost salvation, it would contradict
John 18:9 "That the saying might be fulfilled, which he spake,
Of them which thou gavest me have I lost none."
I don't know if I am given. I believe in Jesus, that He will finish the work He started in me. This battle is surely not down to our abilities, we have divine help.