Since this Snorlax on the bridge is a special kind of Pokemon encounter, it means that like other special encounters with legendaries, they will appear again a while later, or after you beat the elite four, as long as you don't catch them the first time, and instead you defeat them, they defeat you, or you run. (On Kalos route 7's Bulbapedia page, it says it is a special encounter.)
So if you just catch it, it will never appear again, like with the main legendary in each game. Except it's a meaningless Snorlax. So maybe with this one it's slightly different, so that it just gives up coming back once you defeat the E4, because by then you probably won't want to catch it anymore.