1. Because that is the great commission. the great order. the great instruction. Mt 28:18-20 The purpose of the church is for souls to be won to Christ.
The church is the only one commissioned to win souls. Others will build schools, hospitals, etc, but never win souls.
2. We must be sould winners because we are called to do this great work. Matt 22:14
3.This church must be a soul winning because we were created in Christ Jesus for this purpose. Eph 2:10 There is no reason for God to leave us here on earth after salvation at risk of further contamination, but for that there is a work to be done.
4. You must be a soul winner because soul winning gives great joy and energizing Christians. Luk 10:17 When something is not used for its intended purpose, it is misused. Christians who fall into just going to church get frustrated and discouraged, because they are not engaged in the soul winning purpose.