It's not as effective as you would think.
Even if you dispersed a large number of pellets with an explosive charge they would still be in a relatively small cloud traveling in a single orbit - you rely on your target crossing this orbit at right angles to have a reasonable chance of hitting them.

That was the problem with Iridium collision - the Iridium satellites are in a polar orbit (to give a full Earth coverage) the Russian Kosmos was in a nearly equatorial orbit - so every orbit their paths cross (although normally well apart).

The main source of pellets in LEO is from the upper stages of large solid rocket boosters (eg Delta + Ariane) they contain aluminium to control the burn rate. As the motor runs it spits out little balls of molten Al in a trail behind it. In low orbits these decay fairly quickly. The shuttle SRB don't reach orbit.

The Chinese weapons test was particularly annoying because they used a satellite in a mid orbit where it will take a couple of hundred years for the debris to decay. The US test was deliberately done at a very low altitude and any debris would have de-orbited in a year.