I don't have all the sources and grounding for the various suppositions I am going to present so I apologize -- I fear I can't put this question into proper words:
We refrain from eating matzah (at least) on the day time before the seder so that the matzah can be eaten with the proper appetite.
The mitzvot of the second day of yom tov must be kept and in fact (according to the Chabad website) "it was decided that we must be just as strict on all matters on the second days as on the first".
If these are all the case, then how can we be required to eat matzah during the day on the first full day of Yom Tov if it would mean not eating matzah with the same appetite during the second seder? We should at least be required to stop eating matzah by the same time that we stopped eating it on erev Pesach.