Case insensitivity by default on OSX is probably because there are so many important legacy apps that used to (and probably still do) fail spectacularly or subtly when running on a case sensitive file system. Photoshop, Steam, and many more fall or fell into this category. And as long as it remains the default, sloppy app developers won't pay attention to file name casing, perpetuating the situation.

On iOS, there's no such legacy, and Apple decided to make that the default there. Which is something to keep in mind when working with Xcode's iOS Simulator...