The reason is that math and foreign languages are conceived as sequences. It’s as if you stopped after geometry sophomore year and took algebra2 senior year. You’d likely be very very rusty with both algebra and geometry concepts. It’s the same with foreign language: interrupting the sequence makes starting again very rough.
Sciences and social sciences are conceived as self contained units, so it’s not the same for Biology or American History.
You can choose to stop now, and take the full requirement (either in the same language or a totally new one) in college.