[HoneyBadger]

Crossbows were frowned upon as not being the weapon of a Christian gentleman (meaning that it was perfectly OK to skewer a Muslim with one, but quite unchivalrous and rude to knock a proper knight off his noble steed with a crossbow bolt, especially since peasants could do so behind a bush. Peasants, after all, were made for sucking.)

I suspect that infamy had something to do with the rise of gunpowder, which fell under the very popular "knocks over castles" clause.

It's Rule of Cool, for lack of a better term.

Longbows remained quite popular, even after the introduction of gunpowder. Their decline had quite a bit to do with the fact that all the yew trees that made the best bows, had already been harvested. Mature yew trees became remarkably difficult to locate, in the late Middle Ages.

Slings, on the other hand, are insidiously difficult to aim, and fairly dangerous to friendlies, in close quarters. While powerful, they just weren't very good weapons for using in ranks.