Why is congress prohibited from taxing exports

Because the British had done so to them during their rule and the new America’s didn’t want to be anything like the British who treated their people unfairly and with much disrespect.

The Export Clause, found in Article I, Section 9, Clause 5 of the U. S. Constitution, directly states "No Tax or Duty shall be laid on Articles exported from any State." The Clause represents one of the few restrictions on Congress's otherwise broad taxing power.