Why was the Constitution ratified only after the provision for amendments was added?
In large part, the division that emerged at the Constitutional Convention drove the idea of a delayed ratification of the Constitution. While the large debates over slavery and representation were hot button issues with members, there was a much more fundamental and theoretical debate that emerged. The exact role of government was under incredible discussion with two camps emerging. The Federalists were convinced that Shays' Rebellion and the chaos it caused were reasons enough to reject a weak sense of government, in that law and order had to be considered the most fundamental of realities in the new political setting. They were opposed by the anti- Federalists, who sincerely believed that swapping one tyrant political order for a new and homegrown one had to be resolved. They were animated by the belief that the Revolution had only started because of a denial of individual rights. Their main blockage to ratification existed on the level of ensuring that a sphere of political rights had been added, coming in the form of the initial ten amendments to the Constitution. It was this "Bill of Rights" that allowed them to accept the document as a governing one.
The Anti-federalists, led by Thomas Jefferson and James Madison, were pushing for the addition of the Bill of Rights to the Constitution that limited government and police powers and protected free speech and gun rights. Without it, it would be difficult to get the Constitution supported by the anti-feds and ratified by enough states. George Mason in particular led opposition ot the Constitution without these written guarantees of rights, and he nearly kept Virginia from ratifying. This might have been enough by itself to defeat the Constitution's approval.
The amendment provision, and the promise by Federalists George Washington and Ben Franklin that a Bill of Rights would be supported under the new government by both sides, was enough to allow ratification to proceed. They followed through on their promise and added the first ten amendments to the Constitution in 1791 under George Washington's first administration, with a largely Federalist Congress and support from most of the states.