A "consistent design language". The developers who worked on the feature argued against it (among other things it was almost impossible to tell which window was active since "Black on White" isn't easily distinguishable from "Gray on white"). Selfhost feedback said the same thing. It didn't matter - the designers had their mocks, the PMs sided with them, and that was how it shipped.

The public response was entirely predictable (for "not being the target customer", it turns out that we knew how they think pretty well) and soon enough we added a toggle for color and APIs to control title bar colors back in TH1.