I conjecture that it is literature/Engligh. As evidenced on this forum and elsewhere people don't write well in general. Math & Physics is plug and chug. Thats why Indians and Chinese excel at it. Need a thesis? Just rip a paper online, make some minor changes, resubmit, and presto you're a PHD physicist! Literature is far more difficult. You can't fake a novel. You can't fool an English teacher. If you submit a paper with typos you get no points.