Usually, it is because the cartridges they are intended to feed are tapered.
Put 10 or so 7.62x39 rounds against each other, side to side, and see how much it curves. If you try to fight it, they will tend to nose-dive instead of feed.
The MP-5 uses both straight and curved mags. The straight ones came first, then the curved versions were introduced to address feed problems with JHP ammo.