The major difference is that treaties aren't subject to the Presentment Clause. Ordinary legislative acts originate in one of the houses of Congress and must be approved by both houses before being presented to the President. Some congressman or senator is on the hook for having written and sponsored that bill. International treaties are given to the Senate on a take it or leave it basis, and do not require any approval from the most democratic chamber of Congress: the House.

Treaties are undemocratic by design. The framers worried that a democratic republic would not be able to present a unified front internationally. But the purpose of treaties was to govern international relations, not submarine domestic legislation under the guise of international relations.