RN here, it’s because your body has a more difficult time accessing the sugars in fruit form. They are wrapped up in multiple types of fiber which add bulk to your digestive system and serve as kind of a shield for your body to less easily access the sugar. The juice is essentially just the sugar with all the fiber strained out, has others have said this causes a situation where you can drink the calories from like 18 oranges in 45 seconds, and your body can access it immediately and easily. This is also why whole fruit is actually a good thing for diabetics because the glycemic index is actually pretty low.