What we know about 1st century Palestine is that decades after John had died, people were still following him and understanding the events of the world through him. One of the most important documents we have about Judaism in the 1st century gives at least twice the space to John than it gives to Jesus. John was a credible witness because he was so popular and had so many followers. So when John the Baptist says to pay attention to this guy, it would have some urgency.