The first is for the same reason that fairy tales always stop when the heroine's married -- They Don't.
Really. There are fairy tales that go on after the wedding, and even some that start with the heroine -- or hero -- married. And there are fairy tales where the witches aren't wicked. Curiously enough, they overlap somewhat. But the simplest way to meet a not-wicked witch is to marry someone who changes shape. A bridegroom whom you meet in bear form, but who can turn into a man, or a bride whom you meet in frog form, but who can turn into a woman. Then you burn the skin, or violate some other prohibition, and lose your spouse. (Or betrothed -- it's not necessary that it be after the wedding.) Then you get to trudge over miles and miles and miles -- and meet, often enough, three witches. Two can't give you directions except to the next witch, but they will give you a gift that you had better hang onto, and the third will direct you to the right place. (Possibly after consulting all the birds in the world; the first two often failed because they can only consult all the fish in the world, and all the beasts in the world respectively.)
Mind you, it's not a guarantee. You might meet the Sun, the Moon, and the Stars instead (and their respective mothers, often). But that's your best bet. Because they serve a plot function there.
Which, of course, is why the witch is "always" wicked. If she doesn't insist on the daughter as payment for her rampion, doesn't try to eat the children, doesn't take to the storm-tossed sea in her dough trough and paddle up to the princes to demand their young brother as the price of saving them, doesn't impose impossible tasks on the prince, doesn't turn her stepson(s) into swans or a deer -- what happens to the story?