Why did Jesus have sin's flesh?
He didn't, at least not the Jesus of scripture. He came in the likeness of sinful flesh, not that He came in sinful flesh. You are speaking a serious blasphemy here, you know that ?
3For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh
, and for sin, condemned sin in the flesh:
The word likeness here is homoiōma:
that which has been made after the likeness of something
a) a figure, image, likeness, representation
b) likeness i.e. resemblance,
such as amounts almost to equality or identity
Paul says the same thing in another way Here Phil 2:7-8
7But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness
8And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross.
Now since you made such a serious blasphemous statement, its on you to provide the scripture of Truth that states clearly that Christ came in sinful flesh or that He had a sinful flesh.