Why do (either positively or negatively) charged balloons (an insulator) hang on a wall (which is also an insulator) if the wall is neutral?

When a charged balloon is brought near a wall which is neutral, it still sticks to the wall because the charged balloon induces the charge in the neutral body.

If the balloon is positively charged, the electrons in the neutral body will be attracted to the balloon causing an imbalance of charges in the neutral body.

Similarly if the balloon is negatively charged, it will attract the protons of the neutral body and will stick to it.

The phenomenon is called induced charge into the neutral body.