Many of the chief priests and elders did not believe in John's baptism. Thus, they were not prepared to accept Jesus. So they crucified Him.
Many people are so ignorant of history or disobedient to God that they do not take Advent seriously. Thus, they are not prepared to meet Christ at Christmastime. So they miss out on Christmas.
When Christmas is a far cry from what the Lord wants it to be, many do not realize that "the trouble is in their set." Rather, they think their mediocre, watered-down Christmas is the true meaning of Christmas. With this view of Christmas, they do not expect much for the next Christmas. Therefore, they see little need for Advent, since things that do not mean much (like Christmas) do not need much preparation. So they have another poor Advent, leading to another substandard Christmas, leading to a poorer Advent and Christmas next year, etc.
Are you in a descending spiral of Advents and Christmases? This is very bad for your life, death, and afterlife. Throw your heart into Advent now before you miss out on Christmas again.