It’s not one person despite the headline — it’s the Trump administration. The DOJ was prosecuting the case and that’s a federal executive agency, and there is a hierarchy where higher ups can direct where limited prosecutorial resources should be directed.

Note that I am not arguing whether or not this instance is justified or corrupt/BS, just explaining that each administration can direct what cases they want to enforce. You can’t prosecute everything, and there is political judgment involved as to what is a priority.