Because the foolishness of God is wiser than men; and the weakness of God is stronger than men.
The foolishness of God - i:e., God's plan of salvation, which men deem "foolishness" (1 Corinthians 1:21).
Weakness of God - Christ "crucified through weakness" (2 Corinthians 13:4, the stumbling-block of the Jews), yet "living by the power of God." So He perfects strength out of His servants' weakness (1 Corinthians 2:3; 2 Corinthians 12:9). Is wiser ... stronger than men - is not only wiser than men's wisdom, and stronger than men's strength, but is wiser and stronger than men themselves, whatever they have or are (Romans 5:6). It effects revolutions in the individual, and will do so in the whole world, such as man never could.