God doesn't use dirty vessels. If you want the Almighty God to use you for His assignment, then you must first purge yourself clean.
He can't behold sin at all. Sin annoys Him, so how do you expect the Lord to use you for His assignment if you are still sinning?.
Another reason the Lord can't use a dirty person is that whoever He uses becomes His representative and an example for other fellows to follow.
Naturally, the instance people see a fellow performing miracle in the name of Jesus, they usually follow him or her and even emulate the person's lifestyle. If the Almighty God uses a sinner, what message is He passing to the world?. Is not He then saying that it is okay to sin and can go ahead and sin the more?. God forbid.
For this reason, when the Almighty God is looking for a fellow to use, He will first take the fellow through a lot of tests and training just to make sure that the fellow is strong and equipped enough to withstand temptations and sin when He eventually begins using him or her.
Those who are actually serious about the Lord using them have to always subdue the flesh. They learn to walk in the Spirit in the light of the fact if they live by the flesh, they will sin. They fast always, they avoid bad company and social media content that could corrupt their minds. They study the Bible always because they want to know God more.