From my point of view, classical music has a slower and more stable rhythm than a bunch of short pop songs, and is often in Italian rather than English (when there are any lyrics), which (since I do not speak Italian) ensures I do not get disturbed by someone talking in my ear. Also the general noise level of classical music is typically less than pop music.

Now everyone is different and I am sure some people find it more comfortable to focus while listening to heavy metal!