Acrtually, that's not quite right, there are non-Japanese members, although they don't really actively encourage it anymore. There was more outreach in the past - an office in Paris before the war, for example. I think that it's clear that Onisaburo Deguchi believed that he was destined for a role on the international level.
Altogether you have seemed to miss my point. If we were to wonder why O Sensei did not encourage his non-Japanese students to become part of his religion, then the answer lies partly in the fact that the Oomoto kyo did not ecourage this at the time. Even if they may have made exceptions to that rule, it does not change that fact. The fact that Oomoto kyo had been active on reaching out on a bigger scale (Paris, Brasil) around 1925 is not relevant. O Sensei did not have foreign students at that time.