I'm trying to see where and when affusion became commonplace. The NEw Schaff-Herzog enc. says:
"The first extended discussion of the question is found in the epistle of Cyprian to Magnus written about the middle of the third century. Being asked whether those can be deemed legitimi Christiani, "Christians in full standing," who, being converted in sickness are non loti sed perfusi, "not immersed in the water but having it simply poured over them," he gives an affirmative opinion but does so with the very greatest hesitation. His words are: "So far as my poor ability comprehends the matter;" and "I have answered your letter so far as my poor and small ability is capable of doing;" and "So far as in me lies I have shown what I think." He disclaims any intention of saying that other officials should recognize affusion as baptism and even goes so far as to suggest that those who have thus received affusion may on their recovery from sickness be immersed. But, citing various sprinklings in the Mosaic ritual, he gives the view, that necessitate cogente, immersion being out of the question, those who have been poured upon may be comforted by being told that they have been truly baptized ( Cypriani epist., lxxv [lxix], 12-14; A N F, v, 400-401). This epistle makes it clear beyond all controversy that in the third century the ordinary baptism was immersion, and that even in the Latin Church there were those who declared it the only baptism. It further appears with equal clearness that affusion was never practised in the Apostolic Church, for had the apostles resorted thereto even in a single instance Cyprian would certainly have known the fact and would never have presented so mild an apology for a usage which had apostolic precedent, nor indeed would any one have taken exception to the practise. 2. The Testimony of Cyprian.
For a thousand years the resort to the use of affusion was justified only on the ground of necessity. And the supposed necessity existed in the idea that baptism was essential to salvation and so that when immersion, the established rite, was out of the question, something must be put in its place or the soul would be lost. The use of affusion would never have been thought of except for the idea that water baptism was essential to salvation."
So I guess part of the hesitancy from the Orthodox side is that affusion is OK in extremis but should not be used normally. Is it a bit like using some other liquid than wine for communion in a Soviet gulag but this being unacceptable and invalid outside of such extremis circumstances. I'm only thinking out loud.
But there are some references here to theological reasons for affusion not being normal. I'll have to read St Cyprian again.