Alcohol and water are both polar and form hydrogen bonds, but of differing strength. You want those bonds to switch, which creates conformational changes and rips the structure of proteins apart. If you just replace all the bonds with alcohol you'll develop a new, although denatured, stable configuration which can keep the cell wall intact. Instead you want to keep developing new stresses on it until it breaks. Neither can really penetrate the phospholipid membrane of the cell because it's nonpolar, but those conformational changes create big holes where the surface proteins are.