Why does it mention Beis Aharon?
Shocher Tov: The midwives feared Hashem and did not obey Pharaoh; He made Batim for them (Shemos 1:21), i.e. Kehunah and kingship (Shemos Rabah).
Radak #1: They must thank Hashem for David's kingship, for Sha'ul killed Nov Ir ha'Kohanim.
Radak #2: Kehunah (i.e. Avodas ha'Mikdash) was Batel from them for many years, and now it returned to them.
Malbim: Everyone must thank Hashem according to his level. Kohanim received 24 gifts corresponding to 24 Mitzvos of Kehunah 1 .
Beis David received additional benefits - "Beis David
Why does it repeat "Ki l'Olam Chasdo"?
Radak: It says "v'Haysah Lo ul'Zar'o Acharav Bris Kehunas Olam" (Bamidbar 25:13).
