[lbotos]

It's kind of both rarity and usefulness. For example, Black Lotus, is rare but also incredibly useful.

Creatures have mana cost (1 mana, 2 mana etc) in the game. To play creatures you need mana and mana cards can only be placed on the board 1 per turn. this means usually a deck with a some creatures having 1 mana cost so you can hopefully play them turn one, some with a cost of 2 for turn 2 ... etc.

Black Lotus let's you get 3 mana when you play it. So you can put serious pressure if you have a BL because it's basically "turbo" and let's you put bigger creatures on the board "earlier" than normal. (In theory a creature costing 6mana wouldn't come into play until at least turn 6 because of the 1 mana played per turn.) There are also ways to use cards that copy other cards abilities so if you copied BL you could make a lot of mana fast. That "breaks" the game because everyone would start to use that tactic and you'd get a less diverse metagame.

If WoTC made a "red lotus" that did the same exact thing as BL, the value of the Black lotus would drop (pissing off collectors) and they'd have to restrict this red lotus the same way (You can only have one BL per deck in the current rules) or the red lotus would end up more valuable than the black because it wouldn't be restricted to one per deck and people would want 4 of them. (the max of a regular card per deck.) This would also probably piss off current players because it "broke the meta" and would be expensive.

The reserved list isn't the best solution in the world but it's pretty solid to avoid making even more people mad. Most people that have "invested" in a BL are still playing magic or involved in some way so why make them mad?