Why was the legislative veto declared unconstitutional

This legislative veto stands unconstitutional because regarding proceeding 'new law' without having to receive the permission of both houses of Congress and the president.

Explanation:

Congress, being one concerning the three parallel branches of government, is ascribed vital powers on the Constitution. All legislative power under the government is vested under Congress, indicating that it is the single part of that government that can produce new laws or reform existing laws. Executive Branch agencies affect regulations with the specific full force of law, but these remain only under these authorities of laws practiced by Congress. The President may veto proposals Congress declares, but Congress may further reverse a veto by a two-thirds vote under both the Senate and the House concerning Representatives.