It's to avoid people obtaining items that do not exist in future games, e.g. TMs that are no longer in use (and TMs cannot be held items in Gen V anyway), Masterballs, Exp. Shares, basically making the game much easier for yourself.
Many people used to transfer their rare items from Generation to Generation; in Gen V, your Pokemon will be able to be traded while holding an item (that's how many people trade for evo stones), but you can't do that outside of a generation because Nintendo didn't want people stockpiling rare items and then using them in future games to make it easier.
Even items available in Gen V are not transferrable, because the amount of time involved for coding every item would be a waste of effort. You can obtain most items in Gen V easily (Heart Scales are given once a day by a lady in Driftviel if you show her a Pokemon with the move she asks), and those that you cannot obtain more than a few of, the game was designed to be that way.