Harder to manufacture, perhaps? Making tiny finned darts as opposed to solid spheres is more complicated. They probably don’t handle as well either - some guy at the factory dropping a pallet of tungsten balls won’t affect the ballistics much whereas flechettes could be more susceptible to damage.
There is no kill like overkill.
ISTR reading an article that some studies showed flechettes don’t do significantly more damage than ball ammo - the fins actually stabilize them in flesh just as well as they do in air, so you get small wounds. Given that they are more expensive to manufacture, they just went back to spherical shot.
Most dedicated infantry fighting vehicles carry something heavier – the M2 Bradley has a 25 mm autocannon, in addition to 7.62 mm machine guns and rifle ports. Clearly all the major armies of the world have decided that a .50 cal just isn’t good enough for many cases – the 25 mm uses much larger explosive rounds for one thing.
But in the current Iraq war, the Abrams doesn’t have divisions of Soviet tanks to shoot at. It also turns out that the lightly armored IFV is a poor compromise for urban fighting. The M2, for instance, is lightly armored and vulnerable to IEDs and RPGs, but doesn’t even have the mobility of a Humvee. So the Abrams has been used more and more for infantry support, and they developed this canister shot to give it some needed firepower. Two if its machine guns, likewise, are mounted on top of the turret Otherwise the Abrams doesn’t have any round for its main gun that’s effective against personnel.