Because the Shockwaves are designed to be used with magnum charges. The Powerbelt usually works best with around 100 grains of powder. The reason they load so easy is that only the plastic skirt is engaging the rifling when you seat them down the barrel.

If you remove the skirt, you can drop the actual bullet down that barrel of most guns - if the barrelis clean. Upon firing, the heat and pressure of the burning charge causes the lead to obturate, or upset, and fill the lands and grooves. If you ever recover a fired powerbelt, you will find deep groove marks in the bullet - and the bullet is about .008 bigger than it was when you loaded it.

The issue is that if you don't load enough powder, the bullet won't upset enough to grab the rifling. If you load too much, the skirt will disintegrate and allow the gasses to get by the bullet before it seals up the bore.