Q. At Seder we bless God “who commanded us about eating matzah”. Why don’t we say this b’rachah when we eat matzah during the rest of the festival?
A. The basic mitzvah is to eat matzah at the Seder (“in the evening you shall eat unleavened bread” – Ex. 12:18). According to the sages this means that during the rest of the festival matzah is r’shut, “optional” (Pes. 120a).
Though the Torah speaks (Ex. 12:15, 23:15) of eating matzah for seven days (eight in the Diaspora), this is understood as meaning that apart from Seder night there is no duty to eat matzah, although the absolute prohibition of eating chametz still applies (Shulchan Aruch, Orach Chayyim 175:7).
This view is relied upon by some people who cannot tolerate wheat; they eat a basic amount of matzah at Seder but thereafter avoid matzah (unless they can get non-wheat matzot).
Nonetheless many authorities say that matzah during the rest of the festival fulfils a mitzvah. Since there is a question about the legal status of the matzah apart from the Seder, only the hamotzi blessing is said and not al achilat matzah.