Because Microsoft just pissed away their biggest brand after Windows and maybe Microsoft. Brand recognition holds value, a lot of value.
Imagine Pepsi deciding that they are done with Pepsi Max, arguably their biggest brand, after Pepsi itself, and decides that it's now Pepsi Cake. Just kill of all references to their biggest brand. That wouldn't go down well and Microsoft is only getting away with it because pretty much everyone who needs it already have their subscription.
Imagine killing the globally recognized Twitter brand for a single generic letter.
The common denominator is ego. Office may be a globally recognized brand, but it was defined by someone else, and the person making the decision needs to mark their territory.
That's okay because everyone just calls it twitter anyway. there will be kids on twitter, calling it twitter, who were never alive when it was still named twitter
Literally everyone I know calls it "Coke Zero" - in fact, the "Zero" pattern has spread to the various soda companies to reflect the particular style of zero sugar.
I only just now noticed it changed in the US, which apparently happened in 2022. I remember seeing "Coca-Cola sin azúcar" and "Coca-Cola sans sucre" in foreign markets before, I feel like I saw that pre-2022. I don't recall ever seeing a "Coca-Cola cero", for instance. Kind of feels like it was more aligning the brands internationally but maybe I'm just misremembering.
I'm a shareholder and I have a problem with it. It's a bad focus and product strategy over-dosing on AI hype which I think will hurt them in the future.