Why is YHWH called YHWH

If the name given to Moses in Exodus 3:14 was "Ehyeh Asher Ehyeh" and to tell the Israelites that "I AM" has sent him? Then why is he referred to as YHWH meaning "he to be" or "he is"? Why isn't he js called "Ehyeh"? Which was the original name given to Moses?