1. I expect the main reason for needing remedial writing would be English as a second language people. That is, smart people who got a good education, but that education wasn't primarily English. That doesn't really apply to math.

2. I expect standardized tests can more easily test math than writing due to the nature of solving math problems vs writing. Thus Harvard would be able to filter out people who are bad at math more easily than people who are bad at writing. Then again, maybe Harvard doesn't want to filter out people who are bad at math.