Because if Paul were to believe that through his works he could rebuild it, then that would actually be sin and
therefore, Christ then would be the minister of it. He is using it to demonstrate that using one's works, no matter
which works they are in order to become saved is sin. A person can only be justified by Christ, and that comes with being saved by Christ. It is instantaneous with/by/from salvation.
The verses concern a new convert to the faith.
But thanks be to God that, though you used to be slaves to sin, you have come to obey from your heart the pattern of teaching that has now claimed your allegiance.
18 You have been set free from sin and have become slaves to righteousness.
19 I am using an example from everyday life because of your human limitations. Just as you used to offer yourselves as slaves to impurity and to ever-increasing wickedness, so now offer yourselves as slaves to righteousness leading to holiness. Rom6:17-19
Paul tells the romans though they used to be slaves to sin/dead in sin, by following the teaching of grace they crossed over from being slaves of sin to slaves of righteousness leading to holiness. But it takes time, it is not instantaneous to cross over from one state to the other. It is this justification of a believers faith that is mentioned in the quoted verses.
A man accepts Christ as his saviour, he is a drunk, habitual thief, a serial adulterer and constantly uses foul language. He is a slave of sin. He seeks to cross over from this state to being a slave of righteousness leading to holiness. He seeks this justification of his faith, not by obeying the law/striving to defeat the sin, but by faith in Christ, he trusts Christ to get him to where he needs to be. Immediately the affairs stop, stealing stops also and the foul language lessens, so fruits of his new found faith are already being seen. However, drinking is not so easy, he has been its slave for many a year. One night he visits a church, his breath smelling of alcohol, and he heartily joins in the service, praising God for saving him through Christ. What would someone who did not understand the message think? ‘’If this man is saved, doesn’t that mean that Christ is a minister of sin? for here he is joining in the service with his breath smelling of alcohol.’’ However, the man hates the fact he still drinks, but he is entitled to still rejoice in his new found faith despite his imperfections, otherwise he must go around in sackcloth and ashes until he is as pure as the driven snow. He keeps looking to Christ, trusting he is saved because he died for him, and trusting him to get him to where he needs to be. Immediately after Paul asks the question. ‘Is Christ therefore the minister of sin?’’ he answers it. ‘’ God forbid.
18 For if I build again the things which I destroyed, I make myself a transgressor.’’
In other words, if Paul rebuilt the system he had sought so hard to destroy, righteousness of obeying the law, and sought to defeat the sin himself to receive such justification of his faith, he would fail and simply prove he was a transgressor/lawbreaker.