You know, a 152mm gun on a light tank kind of takes the "light" out of "light tank". Plus, the M551 never has been a very popular design, due to a number of fatal flaws:
1. Poorly armored, and extremely vulnerable to mines.
2. The gun is way out of proportion, and the recoil actually sends the tank backwards, plus it jerks the turret violently. If the gunner is holding the turret rotation wheel, he'll get a snapped wrist when the gun fires.
3. Crappy ammo, the shell casings didn't fully disintegrate in humid climates and the hot fragments could set off catastrophic internal explosions. Though if it was included, the flechette round would be a scary feature. Basically, it turned a 152mm gun into a giant shotgun. Run!!!