Because Paris got his butt kicked. (Paris was only saved by the intervention of the gods.)
- The Trojans escaped losing the duel on a technicality--it would have been unwise to reprise that particular match up.
imho, they may even have been hoping for Paris to lose, so they could return Helen and end the war. But it was pre-ordained that Troy would fall, so no getting to avoid that fate over a single duel.
The opening lines of the Iliad explain:
Sing, O goddess, the rage of Achilles son of Peleus, that brought countless ills upon the Achaeans. Many a brave soul did it send hurrying down to Hades, and many a hero did it yield a prey to dogs and crows, for so was the will of Zeus fulfilled.
Iliad, line 1 ff.
- Paris is not honorable; Hector is.
The other is that these are honor/shame based warrior cultures. If Paris was honorable, he'd likely want to reprise the duel with Menelaus. But Paris isn't honorable--he was received in Argos by Menelaus as a guest-friend, enjoying his hospitality until Menelaus goes away, at which point Paris promptly steals his wife.
(This might be due to Paris' upbringing by a shepherd, after the failed infanticide due to exposure. Paris is sort of wild, raised on Mt. Ida, away from the royal court of Troy.)
One might then ask "Why don't the Trojans betray Paris give Helen back?" The answer is surely complicated, but involves family loyalty--Hector may not respect Paris but he won't betray his brother. Helen may also have asked Priam for protection. Lastly, to give back Helen would be to betray Aphrodite, who promised Paris the most beautiful woman in the world.
In some sense, the Fall of Troy is analogous to the fall of Hector. Troy was put in a situation with no way out, but honored it's commitments. Likewise, Hector may have fallen, but his honor was intact.