Your talking as though Orthodox have only just begun missionary work here. While they may have begun with some Native American Indians in this country, the Russian Orthodox wound up with a nation full of Christianized Europeans who conquered the native Indians of this country. Then add to that eastern immigrants. Generally speaking in that sense, you could say the "parents" came to live with the kids and raise them.
Then by your saying, "... a result of the need of such cropping up organically as time passed. " That must mean they were fruitful and multiplied. Did the parents coming to live with the children have such a result? If so it was only within their ethnic group.
I do not think that anyone could have forseen the cost (in the spiritual sense) of such as resulted with the USA.
So then my answer must be, from what you've pointed out, that being there isn't sufficient need, that unless there are enough churches with monasteries and populated with enough Orthodox people, then there is no need for a synod. Bit of a catch 22 situation there.
Out of curiosity how many Orthodox churches and monasteries in Russia equating to "enough faithful" sustain our synod in Russia?