Was there any reason why WotC made it were an Attunement slot was required for an arm blade?
The simple answer is that it's because WotC are incredibly inconsistent about what needs Attunement and what doesn't. Someone whimsically decided that armblades required Attunement, likely without even considering the balance issues involved, and just did it.
Attunement is the simplified 5E version of the item slot system. Seems reasonable to me that an item which physically becomes part of you would take up a "slot".
If that was remotely consistent, that'd be reasonable. However it is not. Instead we have a system where some quite powerful items don't requirement Attunement, including ones that cover part of your body or the like, whereas other items do, for no particularly clear reason. Furthermore, the most consistent feature is that very powerful items almost always do require Attunement, which means it is not closely equivalent to the "slot" system, but rather is a sort of power-limiter, preventing you from being simply covered in the best items (instead you're going to have at most three of "the best", though I think a handful of powerful items have slipped through the net on this, just as many weak ones inexplicably require it).
You can only ever have three items attuned to you. And as noted, most more powerful magic items do require Attunement. So requiring an Attunement slot is giving that item a serious "opportunity cost". Armblades, unless enchanted, are not a powerful magic item. It does not make sense for them to require Attunement for the way it's generally used - to limit power.
Part of this is simply that WotC failed to properly define the purpose of Attunement. Instead of saying "Yo DMs this is essentially a metagame measure to prevent PCs from having too many powerful items and limit certain items to certain classes" (which is what it largely appears to be), they couched it in more mystical and vague terms, and thus the usage is inconsistent, because some writers just slap it randomly on items, and others seem to think any item that is "controlled" needs it, and so on.
On top of this, let me say -
WotC actually screwed up here. Why? Because:
"Additionally, a creature can’t attune to more than one copy of an item. For example, a creature can’t attune to more than one
ring of protection at a time."
So, RAW, you can only have one Armblade. Whereas in the lore, we know some Warforged have two Armblades, often identical ones. Now, you could argue that maybe you could Attune two different weapon-type Armblades (i.e. Longsword and Shortsword or whatever), but even that is dubious RAW. I don't for a second believe that they intended to change Warforged lore, or prevent PCs from having say, two identical Armblades. This was just lazy, thoughtless, not-even-understanding-the-rules usage of Attunement.
There's also this vague bit of text:
"Without becoming attuned to an item that requires attunement, a creature gains only its nonmagical benefits, unless its description states otherwise. For example, a magic shield that requires attunement provides the benefits of a normal shield to a creature not attuned to it, but none of its magical properties."
So this begs the question that, given that normal Armblades have no "magical" properties in the conventional sense (I guess the retraction is arguably one but it is arguable, as it's largely mechanical), do they simply function without Attunement?
