So, then why didn't the Church add the writings of +Symneon the New Theologian and others to the canon of scripture?
“So, then why didn’t the Church add the writings of +Symneon the New Theologian and others to the canon of scripture?”
Because they weren’t writing scripture, they were writing “commentaries” and “instruction manuals”, though I suppose the same could be said for +Paul.
I honestly believe that The Church is quite clear that both scripture and the writings of the Fathers, to the extent they are in the consensus patrum, are inspired by God. What makes NT scripture “scripture” is likely the Apostolic quality, though that raises questions then of why +Paul and not +Ignatius of Antioch or +Clement of Rome, both of whose letters were read for at least a couple of centuries during the Liturgy (the Shepherd too for that matter). Perhaps the distinction is that the canon of the NT and that of the OT read in light of the NT provides literally everything we need for theosis, with the writings of the others, however inspired, merely providing a gloss.
I do know that the Greek Church is very cautious of anything that smacks of a Mohammedan attitude towards the Bible, Bibliolatry, so to speak. The canons of both the NT and the OT are creations of The Church. I don’t think that can be repeated too much, especially nowadays.