Headcanons for why Espa is usually referred to by the nickname instead of just "Espada" by most people including themself:
There is/used to be another "Espada" at the sector and it came in after them so to prevent confusion it became "Espa" instead
When she was a trainee, she was just this number, right, (N3-3108 or just 3108) and at some point when she "earned" the title of a weapon they finally gave her an actual name. Being suddenly told that they "deserve" such a thing didn't really land after years of "you are worthless & replaceable & not even good enough for a name of your own" and so the prospect of having a whole ass designation for itself was almost scary. They dipped their feet into it by using just the "Espa" instead because having a whole name felt like an honor too big. The feeling might or might not have fully faded at this point (at some point when they are feeling particularly worthless their internal narration just switches to "the weapon" instead of using their name) but either way it just stuck.
Maça, the older sibling-figure weapon that looked out for them before he got refurbished years ago, gave it to them as an affectionate nickname and continuing to use it is their way of keeping it alive. They weren't told he died and he might even still be out there somewhere but they have no way of knowing and will probably never see him again either way.