Do Not Use Exterior Paints Inside Your Home!
Is there any reason why I can't use exterior paint on the interior walls?
Yes, there is. Exterior paint contains certain fungicides and UV protective additives that are not approved for interior use. Because of the extreme conditions exterior paints are designed to endure, the strength of the preservative chemicals used may be unhealthy in closed inside areas.
This begs the question, "Why do you want to use exterior paint inside your home?" If you are looking to get some of the qualities of exterior paint, such as mildew or water resistance, there are interior paints and additives that can do the job just fine.
For example, there are special bathroom and kitchen paints that have very durable, eggshell finishes plus tremendous mildew resistance.
There are also additives, known as mildewcides, that are designed for interior use. They can be added to any paint, even flat ceiling paints and texture paints, to make them more "bathroom friendly" and mildew resistant.