What are the main reasons of EOC for why the Protoevangelium of James was not to be included in the canon? What are the main reasons of Eastern Orthodox Church for why the Protoevangelium of James was not to be included in the canon? I was looking for a resource stating those reasons, but couldn't find. Wiki's article on Protoevangelium of James doesn't say anything about that.
While I can't speak on behalf of my EOC brothers, I may be able to offer some insight as to why the Coptic Orthodox Church considers it apocrypha, and for that matter, give some reasons as to why the EOC considers it the same.

There are certain criteria for canonization that each book of the Holy Bible must follow in order to be accepted by the Church as divinely inspired. 

Criteria for Canonization


Apostolicity 
Orthodoxy
Antiquity
Catholicity (Universaility)
Intrisic soudness


The Protoevangelium of St. James is dated to be written in the second to third century. Therefore, he would not have been alive at the time of the writing of this document which brings some question in regards to the apostolicity of the text.

The teachings of the Protoevangelium differ from Orthodoxy in general in that it mentions that Joseph was married prior to his betrothal to St. Mary. This differs from the teachings of the 4 gospels. It is upon this basis that the author builds the idea that James and our Lord Jesus Christ are step-brothers. This idea deviates from Orthodoxy as well as Catholicism and is not accepted. 

As previously mentioned while the text may seem of old age, dating back to the second and third centuries, in comparison to the Gospels, it is not. All four gospels were written prior to the second century and entail none of the mentioned events that occurred in the Protoevangelium of James. A book that was written after the gospels by several hundred years, contains items that none of the 4 evangelists wrote about. While second and third-century texts may seem ancient, in regards to the gospels, they are not.

The Catholicity speaks for itself. The Orthodox church regards the book as Apocrypha, therefore it is not widely accepted by all of the churches as other books of the bible are.

Finally is intrinsic soundness. The Protoevangelium offers some embellishments such as the birth of Jesus in a cave and his hiding in a trough. 

Each one of the New Testament books of the Holy Bible stood in front of these 5 criteria and were canonized. The Protoevangelium stood in front of these same five criteria and even if it was only slightly flawed in one, the church fathers made sure that documents such as these, would not be included in the Holy Bible.

Some sources

Protoevangelium of James in the Bible?

Orthodox Wiki on the Protoevangelium of 

Decretum Gelasianum

Coptic Heritage | James, the brother of the Lord

St. Mary in the Orthodox concept