The same reason that St. Christopher is not on it. The calendar is not a ‘fixed’ one–after all, many people are in the process of becoming saints as we speak (Blessed Teresa of Calcutta), and some have recently (in the last 10 years or so) become saints (St. Pio) and been added to the calendar. But just because a person may not be on the ‘official’ liturgical calendar doesn’t mean they aren’t saints. . .nor that they might not ‘reappear’ (Case in point: My own patron saint was recently put back on the calendar for November 25 after an absence of nearly 40 years as devotions to her have been increasing). Other saints may be celebrated more in other countries (St. Barbara for example, whose feast day is celebrated on December 4 in France but not here in the U.S. except for private devotions or by those parishes named for her, for example).
Should you wish to celebrate privately the feast of St. Joan of Arc-- it is May 30th.