Don't forget that little centuries long jihad thing that put most of the Eastern Empire in Muslim hands and eventually smothered it. That would have an effect on the ability to evangelize, in fact it still does.
" That would have an effect on the ability to evangelize, in fact it still does."
Indeed, Father, it did and does. One might say that we were busy doing other things, like surviving and preserving our Faith and cultures as best we could. 50 years of communism after WWII didn't help either.