anonymity is the reason. If we recieve an official letter, from a party such as a law enforcement agency of some sort, we of course need to comply with it. We usually answer them explaining we are an anonymizer and we are unable to identify who caused the issue at hand because we do not log and we of course can't hand over any data we don't have in the first place. A permanent port might be used to track someone down, at least if any request contains that level of detail. So that can be seen as a security risk.
We currently consider extending the peroid a port can be assigned to a member at the moment, likely after displaying a warning explaining the risks. As there is a multitude of reasons where such a port may come in handy. We are open for ideas.