As Macdonald said, its primarily due to Zimbabwe still being under economic sanctions (however i'm not sure this totally applies anymore? can someone confirm the current state of sanctions on Zimbabwe?)
Specifically - US law makes it exceptionally difficult if not impossible for financial institutions in particular to do business in sanctioned countries - as paypal is an american company originally they choose not to bother as it is much simpler. Other companies like VISA/Mastercard get around things like this by using company registrations in other countries, and due to the fact that there cards are pretty much standard globally. however this brings other sorts of Red Tape and laws that have to be considered
Financial companies also rate the fraud risk very high as well for Zimbabwe - this is why you don't actually get proper VISA credit cards here (only VISA electron).
