And, The Obvious, Racism

Early waves of immigrants tended to be from European countries. They were a little different, and, make no mistake, even little differences could spark xenophobia. These differences were pretty tiny—Irish and Italian immigrants were regarded with suspicion for being Catholic rather than Protestant.

Seriously: "You're a different kind of Christian and I don't like you," was totally a thing.

Chinese people were seen as being a lot different, both physically and culturally. The differences, which aren't any better or worse, were generally viewed as worse. That's how xenophobia works.

Because cheap labor was predominantly coming from a single place, it was easy to conflate one with the other. It was easy to take one reason to dislike the Chinese and then embrace the whole thing. It was no longer about the economics of the situation, but rather about religion, or skin tones, or languages.

Regardless of the motives, the Chinese Exclusion Act is a racist law. It's right there in the description. If a law targets a specific race of people, then it's racist. Had they wanted to concentrate on the economics, they could have. That would have restricted the rights of powerful businessmen, rather than dirt-poor immigrants. You can probably see why they went the way they did.