Why is 3 Maccabees not in the canon while Ezra is?

Third Maccabees is not accepted as canon, so you are mistaken there; only 1 and 2 Maccabees are, along with the book of Ezra, while 3 and 4 Maccabees are not. Is there good reason for that? Yes, there is always good reason. Even the Eastern Orthodox, who claim to follow the first seven councils, are not following what those councils said, because the first seven councils said that 3 and 4 Maccabees are not part of the canon but Ezra is. I have never been able to pin down exactly how the canon information filters down through the centuries with the Eastern Orthodox; it is very elusive.