It should be explained that indeed, Rashi explains (Exodus 9:14): "All My plagues" - we learn from here that the plague of the firstborn is equal to all the plagues. So too the Or HaChaim explains (Exodus 10:11). The Maharal (in the Haggadah) explains that the firstborn represent the 'beginning', which includes all the details, and therefore the plague of the firstborn is equivalent to all the plagues. And Rabbenu Bachya (Exodus 11:4) explains that the plague of the firstborn is the culmination of all the plagues, and in it was the complete acknowledgment of the Egyptians under divine providence.

In light of this, it is understood that when mentioning the plague of the firstborn, all the other plagues are included with it. And the reason why only the plague of the firstborn is mentioned in the Hallel Hagadol and in the piyyut "Dayenu" is explained because its mention includes all the plagues. Indeed, according to their opinion, the plague of the firstborn was not the reason for the Exodus from Egypt, and therefore one who did not mention it truly fulfilled their obligation.