Why didn't the delegates at the Quebec conference just sign the constitution and make confederation official?

The governments that were represented at the Quebec Conference were not governments of independent countries, they were the governments of Britain's North American colonies, with the exception of British Columbia.

They were not breaking away from their mother country in a revolution as the Thirteen Colonies had when they formed the United States; but they were discussing whether they should join and create a new nation and they felt they needed Britain's permission to do so.

The British Government gave its blessings and, with direction from several of the Quebec delegates, John A. Macdonald in particular, they created and passed the British North America Act, which was based on the 72 Resolutions passed at Quebec.