One reason for an ISP to regularly change IP numbers is to save money. If they get a police request regarding a crime, they will then be able to say that "sorry, but we don't know who used that IP at that specific time".

Another common tip spread between the ISPs in some countries is to run very short logs, and to make sure that the logs are not backed up. That will make sure that the ISP will not have to spend many hours of work to retrieve all log backups when requested. Then can do a quick grep through their 48-hour log and email back the few matching lines they found. Problem solved in minutes instead of hours.

In some countries, the above isn't allowed, in which case it is cheaper for the ISP to keep the IP fixed, to make it easier for them when it gets time to extract the logs for one of their customers.