Why was the Constitution ratified only after the provision for amendments was added?

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.