Tosafos on megilla 4a asks this question and gives 2 answers.
That gam haio bioso neis is rabbinic . The hekesh by matza is deoraisa.
Gam haio bioso neis would obligate women deoraisa in matza, however you would learn out from the hekesh 15 15 from Sukka to exempt them. Therefore you need the hekesh from chametz to matza to obligate them.