While I am not Chabad, and I do “brok”, I too was bothered by this question and once heard a fantastic pshat in the name of the Lubavitcher Rebbe (Don’t know which one).
The korban that we bring on the 2nd day of Pesach is the Korban Omer which was barley. Barley in the terms of gemara is “maachal behaima” (animal food). This represents that even though the Jewish nation was born when we came out of mitzrayim, we were still growing until Shavuos which we brought the Korban Shtei HaLechem which is “maachal adom” (human food). The avoda during sefira is to take us further from our animalistic traits and bring us to more humane and Godly traits.
The last day of Pesach is also the first day of second week on sefira. To represent one more step from the basic animalistic nature (of eating anything BASIC just purely for survival) to a more human nature (of enjoying fantastic food) there is an “inyan” to eat gebroks.
Am I totally satisfied with this explanation? Not really. But its some sort of beginning of an explanation. (No one would say that because the last day is miderabanan you should start eating something that is ossur bi’dirabanan!)