One who engages in a mitzvah is exempt from the mitzvah.

Rabbi Michi Shalom!
On the issue of whether a person engaged in a mitzvah is exempt from the mitzvah, Tractate Sukkah, page 25, page 2, the Gemara explains that we learn from the mitzvah of reciting the Shema and also from the mitzvah of the Passover sacrifice, and claims that we must learn this from both. For example, from the Passover sacrifice we learn that it is permissible to bury a mitzvah dead person even at the expense of canceling the mitzvah of the Passover sacrifice, for which cancellation is subject to a kereit. My question is simple. Why? Why fulfill a mitzvah whose punishment is lighter and cancel a mitzvah that is more important. What is the logic?