The general issue is isolation, for sure: you need to be able to cleanup the operation regardless of what state it is in when you abort it, and that means that anything you share with it needs to be designed for this. Threads in most OSs simply don't provide enough isolation to do this, two threads share enough resources that you can't effectively do this (in part because many other parts of the system assume that that threads are not an isolation barrier), wheras processes and the means of sharing things between processes are specifically designed so that this is possible.