This bisque mug has been glazed on the inside. But the bisque has absorbed water from that glaze and this thin walled mug is now water logged as a result (except at the thicker base). It does not have the absorbency needed to build up a thick enough layer of glaze on the outside. Even if it did, the water from the two glazes would wet the bisque so much that its drying time would be greatly extended. This is a problem because the mechanism of attachment of glaze to body is fragile and works best when the glazes dries quickly (if drying is too slow bubbling and cracking can result).
Many ceramic glaze benefits and issues are closely related to the thickness with which the glaze is applied. Many glazes are very sensitive to thickness, so control is needed.