Why do you suppose the join or die wasn't popular at first and became popular later

Benjamin Franklin is arguably one of the most fascinating figures in American history.  He was an author, printer, satirist, political theorist, politician, postmaster, scientist, inventor, civic activist, statesman, diplomat, and not to mention, one of the United States’ most prominent Founding Fathers.
Franklin owned and ran the Pennsylvania Gazette, a Philadelphia-based newspaper that featured the “Join, or Die” cartoon on May 9, 1754. His ability to disseminate powerful messages helped reinforce his influence as a communicator. From an early American propaganda perspective, Franklin was revolutionary.  According to an article in BBC, the Pennsylvania Gazette “is widely considered to be the first American publication to illustrate news stories with cartoons and [the ‘Join, or Die’] political cartoon is believed to be the first of its kind in America.”
During Franklin’s era, there was myth that a severed snake would come back to life if the pieces were put together before sunset.

The cartoon depicts the early American colonies as a snake divided into eight segments.  Toward the head of the snake, “NE” represents New England, followed by “NY” (New York), “NJ” (New Jersey), “P” (Pennsylvania), “M” (Maryland), “V” (Virginia), “NC” (North Carolina) and “SC” (South Carolina).  Even though there were four “New England” colonies, Franklin lumped them into one category to stress the need for colonial unity. At the time, the colonists fiercely debated expanding west of the Appalachian Mountains and fighting the French and their Indian allies.

Democracy and darwinism would be my answer.