The English Constitution was not a written document- it was more informal. When the time came for the Americans to draft their own constitution, they decided to write it down. Why do you think a written constitution seemed like such a good idea? PLEASE HELP

So that they can't change their mind. The early monarchs were known to bend the rules, one of the prime reasons the colonists left, so since they wanted a different type of government it makes sense they would write it down so there wasnt any chance of a new monarchy which they were trying to avoid.