The reason why? The extra hassel and expense.
Generally it requires going to the Post Office, or shipper twice. Once to get an estimate of the shipping costs to provide the buyer, & the second time to actually send the item. Generally when the buyer finds out the additional costs involved he/she wants the buyer to absorb some of it. As as person who works regular business hours, it's not easy to get to the post office for all of this. The 30 seconds of paperwork, spoken of above might be that, if you are experienced at filling it out, but might be 30 minutes if you haven't done it before, or do it infrequently. Helpful post office advice is not always easy to come by. Then you have to worry for the next couple of weeks whether the package will actually get there and in what condition, while to buyer keeps asking when it will arrive and starts to doubt whether it was actually shipped not?
When you consider there are generally many willing domestic buyers at hand and all of this can be avoided, why bother?
I hope this helps to shed some light on why many do not want to ship internationally.
- augustwest