Crystalline solids have repeated similar arrangement. But the unit cell which gets repeated has different arrangement of atoms in X, Y, Z planes within itself. If this arrangement is same in all the planes then even crystalline solids can become isotropic.
In case of amorphous solids, the arrangement of atoms is irregular in all the planes whether we view in any direction. Hence same properties are seen in every direction. Hence amorphous solids are isotropic in nature.