The correct answer to this open question is the following.
I think Jefferson's support was so important to Virginia's ratification of the Constitution because he had previously participated with some constitutional writings in Virginia, although he could not participate in the Virginia state convention of 1776. However, his constitutional ideas had major influences on his state. He was a prominent figure of respect and has been the political mentor of James Madison.
Thomas Jefferson wrote one important book called "Notes on the State of Virginia," which included valuable lessons on constitutional issues, and these ideas permeated in Virginia.
Thomas Jefferson ended up being the main figure who drafted the Declaration of Independence, promulgated on July 4, 1776.
