Why cant we peel Banana when it is raw?

Fruits are classified into two groups.

• One, climatric and 
• non-climatric.

Climatric types are those that ripen even after harvest, for example, banana. 

The non-climatric types do not ripen after harvest, for example, grapes.

The climatric fruits like banana need an external assistance for ripening them. Scientifically after harvest ethylene gas is used in godowns to hasten banana ripening. While on the plant ethylene hormone is released naturally by the plant to ripen the fruit.

Raw banana fruit is firm and not peelable since the cell cementing material is made of non soluble calcium pectin. On ripening the non soluble pectin will become soluble and hence the softness. 

And also before ripening the “starch” a complex sugar in banana will transform into simple sugars on ripening thus creating inter cellular void which makes peeling easier.