Why is it that some college or university students who are good at math suck at writing 2000 word essays and papers?

Statistically speaking, the laws of probability suggest that in any group or cohort if a way-of-being is possible it will occur randomly within the given population.

If it is possible that SOME of the students can be good at math but "suck" at writing essays, then given a large enough sample size, some will.

As to why some are good at some things and not others, generally speaking it probably has to do with experience, practice, natural aptitude, and societal and cultural constraints. In the work on stereotype threat we have seen that some people do well (or worse) in some school subjects because of expectations put upon a sub-group of the population. For example, say it is a belief that males do well in math and females do with in writing. Given this constraint, a male might feel stereotype threat when writing a 2000 Word essay. That is just an example.