I think it’s better to to think of USD (and USDZ) as a format for use in art/development pipelines, and for teams to collaboratively edit the same scenes. USD was not originally designed to be published to the web, and building a loader in three.js is far less practical than loading glTF for reasons described elsewhere. By contrast, glTF is designed specifically for publishing and transmission, and is popular for use on the web for that reason.
There have been examples built using USD on the web, but the WASM library required to parse it is much larger than the entire three.js library — so I think it remains a format for a different use case than glTF for the foreseeable future.
Disclaimer: I’m a contributor to glTF, THREE.GLTFLoader, and three.js.