פירוש על פסחים 198:4

A little before Mincha

So that you will be hungry when eating matzah, which is a better way of performing the mitzvah.

...One may not eat: So that he will eat the matzah that was commanded with a hearty appetite, so as to glorify the commandment.

Because of the obligation to eat matzah- so that you do not eat anything before the biblical obligation to eat matzah. As it says in shmot perek 12, "on the eve( of the first night) you shall eat matzah"