Simply just because organic cotton is more expensive to produce. Not using pesticides or any chemicals in the process, the crops generate less yield than conventionally grown cotton. So basically it is just a problem of demand versus supply as there is less organic cotton available than regular one thus driving up the price.
Plus, conventional tampons also have a lot of synthetically produced ingredients which are – as we all know – cheap. All these factors together are to be seen in the slightly higher price of organic period products.