Twitter limits; and also because it's not just marketing Kool-Aid. It's "Word for iPad," "Angry Birds for Android," "Kindle for Windows" and so forth. Not "Word for the iPad," "Angry Birds for the Android," or "Kindle for the Windows."
For iPadOS makes sense. For iPad less so.
Unfortunately or fortunately depending on your perspective, the only method to acquire iPadOS is to use an iPad - so the designation seems fitting.