The reason God decreed that the gospel would obtain people from every tribe and people and nation is that the aim of the gospel is the glorification of his grace and this ingathering of diverse peoples into one Christ-exalting, unified people who would glorify the power and beauty of his grace more than if he had done things another way. There is a strong confirmation of this in noticing that several texts which command the pursuit of all ethnic groups are explicit that this pursuit is for the glory of Christ.
For example, in Romans 1:5, Paul says that his apostleship was given "to bring about the obedience of faith for the sake of [Christ's] name among all the nations." In other words, the pursuit of "all nations" (all ethnic groups) is for the glory of Christ. Similarly in Romans 15:9, Paul says that Christ did his own missionary work in coming into the world "in order that the Gentiles [or nations] might glorify God for his mercy." The aim of Christ's pursuit of the Gentiles (the ethnically different ones) is for the glory of God's mercy, which was shown supremely in the death of Christ.
Accordingly, the consummation of the missionary mandate to make disciples of all nations (Matthew 28:19) is described in Revelation 5:9 as persons from "every tribe and language and people and nation" worshiping the Lamb and declaring the infinite worth of his glory. So the apostolic vocation (Romans 1:5) and the messianic example of Christ (Romans 15:9) and the consummation of all missions (Revelation 5:9) have one explicit aim: to display the glory of Christ through the ingathering of a hugely diverse and unified redeemed people.