I imagine this plays an important part. From my limited experience practicing Buddhism I have detected that many Americans of European ancestry who adopt Buddhist practice as an adult do so after having formulated what in the U.S. are typically referred to as liberal political positions, particularly as they pertain to so-called social issues (as opposed to economic issues).
On the other hand, Buddhists of Asian ancestry in the United States, particularly first-generation immigrants, likely hold more, shall we say, “traditional” or “conservative” positions on social issues. The same might be true (just speculating) with regards to American-born Muslims of African ancestry who have converted to Islam from Christianity, and American Muslims of Middle Eastern, North African, Sub-Saharan African (not counting Americans whose ancestors were here before the middle of the nineteenth century), or Southeast Asian ancestry who are more recent immigrants to the country.
It is notable that some of the religions in the Pew poll are broken down into sects while others are not. For example, Baptists were divided into three groups (National Baptist Convention, American Baptist Churches USA, and Southern Baptist Convention) while other religions were polled as one group. For example, Jewish Americans were polled as a single group, although on “social” issues such as abortion there can be wide-ranging disagreements among different Jewish traditions such as Orthodox, Conservative, and Reform. I suspect the same may be true with regards to other religions in the poll such as Orthodox Christian which is not broken down into the various regional churches of the Orthodox communion such as Russian and Greek Orthodox. If all the Protestant groups had been lumped together the poll would have yielded results with fewer finer distinctions.
Back to American Buddhists, generational issues likely also are at play. I would hazard a guess that Buddhists of Asian ancestry who immigrated to the United States likely hold more socially conservative positions than their offspring, particularly among recent immigrants. I don’t know this for certain, but it would not surprise me given what I know about the political positions of various demographic groups in the United States.