Because, it was the only valid format string in DB2 V9.1 for LUW.
Note: TO_CHAR is a synonym for VARCHAR_FORMAT.
A character constant that contains a template for how the result is to be formatted. The length of the format string must not be greater than 254 (SQLSTATE 42815). Leading and trailing blanks are removed from format-string, and the resulting substring must be a valid template for a timestamp value (SQLSTATE 42815). The content of format-string can be specified in mixed case.
Valid format strings are:
where YYYY represents a 4-digit year value; MM represents a 2-digit month value (01-12; January=01); DD represents a 2-digit day of the month value (01-31); HH24 represents a 2-digit hour of the day value (00-24; If the hour is 24, the minutes and seconds values are zero.); MI represents a 2-digit minute value (00-59); and SS represents a 2-digit seconds value (00-59).