Two factors come to head:
1. Lack of police involvement and understanding of IT. Stealing a phone is a crime which police must be found at the forefront driving force to stop it. They should be exploring ways and means of getting it under control effectively stopping it from spreading and working with ISPs is one assured way to stop it, but its not happening.
2. Morality. Our society is now full of corruption to the extend that people are no longer ashamed to be seen buying and selling stolen goods. If you buy stolen goods because they are cheap, how then are you going to stop thieves from stealing. To them its a business being driven by demand.
On the other hand, our ISPs always want to be seen as socially responsible. If there is enough public outcry about phone being stolen and used on certain networks freely, that network provider will be forced stop it because they might not want to be seen as associating with thieves or criminal activities which can be bad for there image,