Thomas Jefferson, who signed the declaration of independence, was still in Paris working as the French minister during the constitutional convention, which is political position. John Adams was still in Europe serving as the minister to Great Britain.
Many other people did not sign. Samuel Adams, John Hancock (he did sign the declaration of independence) and Patrick Henry turned down an invitation to attend because they thought it was going to result in another monarchy, just like the one they were running away from in England.
Only 39 out of the 55 people attending signed it.