It's called data aggregation. OU wants to keep track of what you unlist, when you unlist, if the unlisted item was sold outside of OU,etc.
For instance, some users bypass buying a bump by unlisting an item and relisting it,etc. They also want to keep track of this behavior as well.
OU uses AWS, so the additional storage cost is minimal.
You answered your own question. In order to keep track of whether or not you will ever list it again on OU is one of the reasons.
If you elect to have a delete option, they can't track all user stats in real time. I was just giving possible reasons for not allowing a delete option.
For instance, there are more than a few unscrupulous users that mark all their sold items as "unlisted", rather than "sold" since they don't want any possible bad feedback from selling items in questionable/hidden conditions.
In any case, neiher Elin or MJ will give you a straight answer. You can try asking "thisisflea", but I highly doubt you will get a suitable answer since we are discussing metadata.
There just simply needs to be a DELETE option. I don't care if you keep your archive featureI personally do not want my sold items floating around on someones database period. Every legit site out there has a delete feature. Get with the times. Please make a delete option or I will simply stop using it and tell everyone I know about how crazy this is on OU.