Back in my day we called it Db. What's next, B## tuning?
Yes that makes sense when you start in E standard tuning and tune every string flat 3 half steps relative to each other.
I think it's because calling it Db would include Cb and Fb which a lot of guitarists would scratch their heads at. C# isn't as puzzling with C# F# B E G# c# . My tuners default to sharps not flats as well probably biased towards the common major keys with sharps.