When we start to talk about things like “dominant understandings,” and why they come to be the way they are, it is very hard to give definitive answers. It is impossible to know exactly what motivates people to see a given group in a given way at a given time. We can only speculate, using what evidence we have. Let us look at four main potential reasons why Asians could come to be excluded from dominant understandings of what it meant to be American.
One reason was economic. Asians were typically brought to the United States specifically for work purposes. This meant that it was easy for them to be seen as an economic threat to those already in the country. It was possible (as it is now with illegal immigrants) to tell a plausible story about how the Asians were taking jobs from the Americans.
A second reason was “racial.” Particularly at the time when Asians were first coming in large numbers to the United States, the US was a country that was very conscious of race. There were many scientific theories at the time that “demonstrated” how human beings existed on a racial hierarchy, with Northern and Western Europeans at the top and “colored” races at the bottom. The physical differences in appearance between Asians and white Americans were a clear reason, in the thinking of the time, to differentiate Asians from “real” Americans.
A third reason was religious. Again, this was a time when American attitudes were much less tolerant. The Japanese and Chinese were, of course, not Christian for the most part. Filipinos who came were Christian, but they were Catholic. Religious differences made all of these Asians seem less “American” than white, Protestant, Americans.
Finally, there were cultural differences. Asians came, of course, from a non-European cultural background. While the US had assimilated various sorts of Europeans, it had not tried to assimilate any non-Europeans other than the enslaved Africans. It had never had to deal with the idea of large numbers of people who were free, but who were so culturally different. These cultural differences made Asians seem not to be real Americans.
In short, what we can really say is that Asians seemed too different in too many ways at a time when the United States was nowhere near as tolerant as it is today. This led to such things as the Chinese Exclusion Act of the 1880s and other laws that discriminated against Asians and treated them as fundamentally different from “real” Americans.