Because Jesus became man,
Is there a law against becoming a man? Did Adam sin by becoming a man? Was it not God who made Adam a man? Was it not God who was responsible for the incarnation of Christ? This could only be a just basis for condemnation under the law if Christ by becoming a man became a sinner?
because he was made a curse for us
Are your suggesting that the Law could justly charge the person of Christ with sin because He was hung on a tree? Thus he actually and personally became a sinner himself under law and thereby God was just in administering the condemnation of the law to Christ upon the cross? If so, then he personally deserved it as he himself was the violator of the law as it was his body that hung on the tree and not mine or yours? Is that what you are saying? It seems to be! You are saying the law found Christ to be a sinner and personally cupable and justly condemnable by Law.
However,if that be the case, then how could it be "for us" as we were already condemned under law and already personally culpable and justly condemnable by Law? It would seem making him to be a sinner under law eliminates him as a qualified redeemer rather than qualfies him as a redeemer from the law.
Moreover, by simply quoting "for us" begs the entire question as to what that really means. Indeed, any man that is hung on a tree is "cursed by God" whether he is sinless or a sinner but not "for us" but only for himself.
The only possible way "for us" makes any sense at all in that context is that he acted not personal capacity but in a representative capacity "for us" in being made sin as the Second Adam, just as Adam acted in a representative capacity and not in a personal capacity in the Garden test. If he had acted personally then he alone would have been charged with sin.
He was "made" to be sin, made a curse by the law. He did not nail himself to the cross. He did not share any responsibility and thus any personal accountability for being on the cross as he was completely passive in this act. This passage says nothing more or less than "he was made to be sin for us who knew no sin." The only possible legal way He could be made sin "for us" by being hung on a tree is if he acted in a substitutionary representative capacity on the cross being condemned in our place suffering vicariously rather than acting as an individual. If he acted as an individual then he justly deserved death. If he acted in a representative capacity then we justly deserved condemnation and satisfied it completely in him.
So claiming he personally deserved condemnation under the law repudiates his qualifications to be a redeemer from the curse of the law as it makes him personally guilty. However, to claim that he acted in a substitutionary representative capacity on the cross and in that capacity he was "made to be sin" is in keeping with his Second Adamic covenant role and does no violence to his own personal righteousness.
The difference between the representative roles of the first and Second Adam are many. For example, the representative role of the first Adam was a test of obedience, which he failed as a person and as a representative. However, Christ never failed in the test of personal obedience. On the cross he was not acting in the role of obedience, but in the representative role as a sinner justly condemned because he represented justly condemned sinners.