Why was there a need to create a new constitution?

if you are talking about current time the answer is no

if you are talking about when the colonies were developing the answer is yes

they believed democracy was worth the fight

the monroe doctrine is the best known u.s. policy toward the western hemisphere. buried in a routine annual message delivered to congress by president james monroe in december 1823, the doctrine warns european nations that the united states would not tolerate further colonization or puppet monarchs.