Why are there musafos (things added to the chazaras hashatz of musaf) for Shabas parashas sh'kalim and hachodesh but not zachor or para? (Or, at least, sidurim, including Rav Yaakov Emden's, have the former but not the latter.)

Why are there musafos (things added to the chazaras hashatz of musaf) for Shabas parashas sh'kalim and hachodesh but not zachor or para? (Or, at least, many nusach ashkenaz and nusach sefard sidurim, including Rav Yaakov Emden's, have the former but not the latter.)

Why are there musafos for Shabas parashas sh'kalim and hachodesh but not zachor or para? (Or, at least, sidurim, including Rav Yaakov Emden's, have the former but not the latter.)

Why are there musafos (things added to the chazaras hashatz of musaf) for Shabas parashas sh'kalim and hachodesh but not zachor or para? (Or, at least, sidurim, including Rav Yaakov Emden's, have the former but not the latter.)