Why is it that Zechariah son of Jehoiada (2 Chronicles 24) not included in the list of 55 Jewish prophets, although he recorded by name?

Rabbi Shimon Keira [of 7th century] in his book Halachot Gedolot [aka Bahag] Siman 76 p.689, based on the Seder Olam, lists all forty eight prophets and seven prophetesses, one of which is Zecharia ben Yehoyada. I don't know why Rashi [Megilah 14a] does not list him.