Because when you use it once, you lose the entire magazine. You can't use a 100-round twice on a 50-round weapon. Plus you have to factor in weight/vol. Why carry something the size of a 100-round if you can fit twice as many 50s in the same space?
The main advantage is if you're carrying weapons that take different ammo sizes, in which case you might find it beneficial to carry only the ammo size for one of them (either the larger cells, for convenience, or the smaller ones if it significantly increases the number you can carry in proportion to the larger).
____________"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
Only fools and children dream of heroes.