I'll just leave this here.

Yoshida: The biggest reason that we decided to go through with this big change to the items and have all of these older items become dated items and add a set of new items, was to accompany the major changes that we made to the battle system. We have all of these changes to the battle system and the items that existed really didn’t fit with the changes that we had made and they were breaking the balance, whether it be the parameters or the restrictions on the items and whether they could be equipped by these classes or these classes. If we were to have kept these original items with this new battle system, it would have created a lot of confusion not just with new players but also with players that have been playing the game for a long time. So to remove that confusion and better keep the balance with the new battle system, we thought the best way to do this was by creating the new items.

One of the biggest reasons for not being able to put Materia onto these dated items is that the minute you’re able to put Materia onto them, it increases that confusion we wanted to avoid because the item will remain with its stats and parameters that don’t match up to the new battle system. So by not being able to attach Materia, it becomes a way of phasing out those items. We’ll also be removing the materials that create those items out of the game gradually as well as removing the recipes that depend on those items. Our aim is to have them replaced by a new set of items. But the thing is, we understand that a lot of players may have been using an item for many months up until now and they really like it. You’re not able to put Materia on it, but you are able to turn it into Materia. So that item that you’ve used, you turn that into Materia and put that on your new item. So in a sense that old item you used is now a part of your new item and that’s what we’re aiming for.
Source:
http://ffxiv.gamerescape.com/2011/10...ere/#more-9022