In short, because we can’t find them in the New Testament. There is no command from Jesus to use instruments in worship. There are no examples of Christians in the NT using them in their worship. Mentions of instruments in the Old Testament (such as in the Psalms) are not our standard today (we don’t continue to offer animal sacrifices either). Whenever we do see music mentioned in the context of worship within the New Testament the Bible only mentions singing (cf. Ephesians 5:18-19; Colossians 3:16-17). Just reading the New Testament by itself, with no prior assumptions, leaves us with no indication that God has asked for, wants, or accepts anything musically other than our singing together to Him.