Q: Why can't I delete iBooks app on my new iPad?

We are fellow users on these forums, as far as I know Apple haven't said why it's now a built-in app, and speculation about Apple's decisions/policies isn't allowed on here.

If you wait until iOS 10 is released in the Autumn (the actual date hasn't been released yet), and if it makes it into the released version, you should be able to delete it : Remove built-in apps from the Home screen on your iOS device with iOS 10 beta - Apple Support

(That link also says 'The apps built into iOS are designed to be very space efficient, so all of them together use less than 150MB. ', so they aren't using up that much space.)