Why didn’t Rambam mention the halacha that one's life takes priority of that of others

Igros Moshe (Y.D I #145) Question Why didn’t Rambam  mention the halacha that one's life takes priority of that of others even though the halacha follows Rabbi Akiva and Rabbi Eliezar does not disagree? Furthermore we need to understand Ben Petura’s view citing no verse which says it is best that they share the jug of water and both die.  This is difficult because only to avoid killing someone do we say it is better to be killed which is a clear act to avoid doing an act of killing. In contrast if he doesn’t have to actively kill but is being thrown on a baby he doesn’t have to die instead. This is because we have a principle “ Who says the other’s life is more important than yours?” So why does Ben Petura say that you must be willing to die of thirst because of a mitzva to give charity to the needy or even to save a life?   Rabbi Akiva learns from the verse that when a person himself needs something there is no need to give it to someone else who needs it. Ben Petura just views that even when a person needs something even to save his life, he is still obligated to give it to someone else who also needs it.