The church recognizes saints by proclaiming their heroic virtue and fidelity to God's grace. When the church canonizes saints, she proposes them as models and intercessors.
By canonizing some of the faithful, i.e., by solemnly proclaiming that they practiced heroic virtue and lived in fidelity to God's grace, the Church recognizes the power of the Spirit of holiness within her and sustains the hope of believers by proposing the saints to them as models and intercessors. CCC 828.
Some angels have been recognized as saints because of their heroic virtue and fidelity to God's grace, and these angels can also fulfill the role of model and intercessor. One particular example is Saint Michael the Archangel, whom we know about from the book of Revelation.
Not all angels are saints because not all angels have heroic virtue and fidelity to God's grace. (Fallen angels are an example of this.) Also, while many angels are in heaven, there are few angels who are saints because we do not know enough about them to canonize them. Saint Michael is one of the few angels that we do know a lot about.
Also note that while we believe all saints are in heaven, simply being in heaven does not mean that a person is recognized as a saint.
