They donít ďsilenceĒ weapons that effectively - Despite what you see in films, the best quality firearms suppressors shave only 20-40 dB off of the sound of a firearm discharging. Itís often not worth the time,nor the effort to obtain one since it doesnít suppress the sound that well.
They are expensive - Sound suppressors purchased legally arenít cheap; those bought on the street are sold for very high amounts of money. Most criminals steal the weapons they use or they purchase them stolen from street sources. A weapon is a high dollar acquisition, leaving little for accessories.
Weapons rarely have integral suppressors - Most of the time the suppressor has to be attached prior to the weapon being fired. Since that takes time and most street shootings arenít well-planned affairs, it would be impractical for criminals to carry or use suppressors.
It brings higher sentences - Using a firearm with a suppressor to commit to crimes is a federal offense in the United States. Also, possessing one without the proper documentation can result in a ten year sentence. Most criminals want to avoid getting harsh penalties and if they used a suppressor, they would likely do so in situations where there would no possible chance that they would be caught doing as such.