Grenadiers as I understand them were initially formed from the most physically imposing and strong men who could throw grenades the furthest, and after the grenades stopped being used they just became more of shocktroops and elites. But why'd they stop using grenades? They seem like a pretty effective tool for ripping up ranks of infantry.
An early grenade is a small, hollow ball of iron filled with shot and powder. Due to irregularities in powder, the ball might explode quicker or take more time, this being a fickle weapon. Further, these are items that are thrown, as you mentioned, meaning that the soldiers would have to get relatively close to throw a heavy piece of iron, close enough for musket fire to be accurate. All of this would combine to make dead Grenadiers.
Grenades were used in siege warfare but only because they were more practical to be thrown from a fort wall than in front of an enemy line.
Edit: fixed errors