Why is MCE suing?

MCE is suing because these standards are ineffective at reducing nitrogen and phosphorus pollution and do not protect human health.
In 2011, EPA denied this set of standards because they were not based on sound scientific rationale and they failed to demonstrate that they would actually be protective of aquatic life and recreation. [LINK 1, PG. 27-28]
Under a new administration, EPA reversed its 2011 decision and approved substantially the same set of standards in 2018.
DNR’s lake nutrient criteria set a bad precedent for streams and rivers, neither of which currently have standards.