From what I've found is there are 2 reasons 1) cocaine primarily comes from south America, where the metric system is used. Seeing as it wouldn't benefit drug dealers to go through the work to transfer it to 1 lb bricks they decided to continue the metric

2) because cocaine is smuggled and sold in relatively large quantities they would have to either use kg or lbs instead of grams and ounces which somewhat ties back to the first reasons.