Why were certain writings like The Shepherd of Hermas and The Didache excluded from the New Testament despite being popular in early Christian communities?
We have to differentiate what the Bible is, versus what those documents are. The key is what is instructive and what is authoritative?
The Bible is the “Authoritative” Word of God. Meaning that it’s documents reveal inspiration by the Holy Spirit for the authoritative guidance and edification of believers.
The Didache and other early documents, such as the early versions of the Apostle’s Creed are instructive. They have value in that they show us what the early church read, shared, believed and accepted as true doctrines of the church. However, they are not Authoritative.
The scriptures that made it into the New Testament are, like the writings of the Old Testament prophets, actually authoritative, because they reveal inspiration by the Holy Spirit.
The instructive writings have value, but only the Bible is authoritatively the Word of God.
Here is summary of the requirements for how the Christian “Canon of Scripture” was concluded at the beginning:
1) Was the book written by a prophet of God?
2) Was the writer authenticated by miracles to confirm his message?
3) Does the book tell the truth about God, with no falsehood or contradiction?
4) Does the book evince a divine capacity to transform lives?
5) Was the book accepted as God’s Word by the people to whom it was first delivered?