Another interpretation: "These are the servants of the Lord." These refer to the people of Israel, as it is said, "For they are My servants, the children of Israel" (Leviticus 25:55), and likewise it says, "Sing and rejoice, O daughter of Zion" (Isaiah 12:6). Another interpretation: "Hallelujah." Why doesn't it say "Praise the Lord"?
Rabbi Yirmiyah said in the name of Rabbi Elazar, "This world is not worthy to praise the Lord in His entirety, but only in part, as it is said, 'Let every soul praise the Lord' (Psalm 150:6). Therefore, it says 'Hallelujah.' From here we learn that Hallel must have at least three verses, as it says, 'Hallelujah, praise God' (Psalm 146:1), which has two verses, and 'Hallelujah, praise God in His sanctuary' (Psalm 150:1), which has three verses.
And one who says only one verse, it is as if he did not say Hallel at all. And similarly, it says, 'For I will proclaim the name of the Lord; ascribe greatness to our God' (Deuteronomy 32:3), which has one verse, and 'Sing to the Lord a new song' (Psalm 96:1), which has three verses. Therefore, it says 'Hallelujah.' Where do we learn that there were Hallel songs when Israel descended to Egypt, and there were 71 with the Divine Presence among them, as it says, "I will go down with thee into Egypt" (Genesis 46:4)?
And it is written, "Thy fathers went down into Egypt with seventy persons" (Deuteronomy 10:22). And when the Holy One, blessed be He, ascended with them, it says, "I will also surely bring thee up again" (Genesis 46:4). Therefore, it says 'Hallelujah' for the number of 'Hallelujah.'