well....im not really knowledgeable on this, but i would use this line of reasoning, if i were asked to argue for that point.
First, we agree that classical music.......is well.......european. almost all well known, even less known, classical, romatnic, baroque, are all european. bach to rachmaninoff to ravel to scharwenka.
These great composers, were (in most cases) also great pianists. There are also the great musicians, not just composers, of the time, such as richter, backhaus, gilels, ziloti(who i think has done some composing), arrau, neuhas, etc, etc.
Lets say that these people and more, are the foundations of classical music. They hold all the knowledge of the music itself, instruments, interpretation, etc, etc.
These people also teach the next generation of musicians what they know about music, they teach them all the things that made them the admired musicians they are.
Then how does this knowledge, and everything about this music, transpose over to north america? The most obvious way is by students of the "next generation" i spoke of, learning in europe, and moving to north america. Now one considers just how many of them actually come to north america, most of them come to perform, then retired in their "teaching" age back to their homeland. some stay and teach, but those few students that they manage to teach (relative to the number of musicians and music schools in north america) are very little indeed.
anyway, my argument would be along those lines. But then again , i dont know much about this education of music business anyway.