The reserved list is from way, way back. In the early days of Magic, they did a set called Chronicles, in which they reprinted a whole bunch of cards--and they reprinted them in massive quantity*, far exceeding the original print runs. As a result, many highly collectible cards tanked in value, and hobby shop owners went ballistic. Suddenly whole shelves full of $20 cards were worthless. (This was back when $20 was considered a high price for a Magic card.)
In order to rebuild trust with distributors and collectors, Wizards came up with the idea of a list of "reserved" cards, which they swore never ever to reprint. Furthermore, they made additional rules: Of the rarest cards in any set, no more than 25% would ever be reprinted, and there would only be one window (the next core set) in which that reprinting could happen. Any card which missed that window went on the reserved list forever.
This plan worked and revived M:tG as a collector item. But as the player base expanded, the reserved list became more and more of a problem. Prices of cards on the list went through the roof, and there was no way to balance supply with demand. Wizards decided in 2002 that no more cards would go on the list.
However, even today, Wizards considers themselves bound by their promise: Cards already on the list cannot be reprinted in any form. They can't even print a card with the same mechanics and give it a new name.
* Massive by the standards of the time, at least. It wouldn't even be a blip today.
