How did the Magic the Gathering designers keep players interested in buying new cards?

The cards have always been split into 5 colors (well 6 originally, since early on, colorless was another color), which have always been roughly equal in power. There's really three ways they have keep people buying new cards:

1) New mechanics. In the early days of the game. You cast (what are now termed) sorceries and instants, which when they completed went to the Graveyard, and you cast creatures, artifact and enchantments which go to the battlefield until they are destroyed, at which point they go to the graveyard. The two ways to win were to take your opponent's life down to 0, or force your opponent to draw from a library (deck) that is empty. Cards in the Graveyard were basically there forever.

Now, there are abilities that let you play certain cards from your graveyard, a variety of ways to protect things on the battlefield from being destroyed, and ways around those, an entire new zone of play called the Exile Zone, which is practically a second Graveyard that is harder to get cards out of. Creatures that have effects as they enter or leave the battlefield. Planeswalkers, an entirely new type of card. They've also added cards with abilities that cause a player to win or lose immediately, and Poison Counters, an alternate form of damage that can kill a player without either of the previously-existing win conditions.

2) Power creep. Early on in Magic, the creators really didn't have a strong concept of what made a card good, so the power of cards was wildly out of balance. Some cards were ridiculously good, where if one player had one and the other player didn't, the one with them would win a lot.

Since then, they've gotten a better control of how powerful a card they print is, and have gradually made newer cards on average a little more powerful than older cards. So if you want to keep your deck up to stuff power-wise with everyone else, you have to routinely update it with upgraded versions of the cards you have in it already.

3) Game Formats. As Magic started to be taken seriously and they got a larger and larger pool of cards printed, it became increasingly difficult to come up with new cards to get people to buy without making card combinations with existing cards that would just be too powerful.

In response to this, Formats were created. The game format determines what cards can be played legally in the game, and occasionally providing an additional limit on the number of copies of a card that can be included in your deck. Standard Format is the most restrictive, including only cards that have been printed in the last 1-2 years (and older versions of any such card). Standard is also the most comonly played format for officially sanctioned games, accounting for over 75% of such games. With that being the case, significant numbers of cards will only be playable in the majority of official games for less than 2 years, forcing the purchase of new, powerful cards.

Yes, the Color Pie was originally part of the game (though the rules were not as well established today), in fact, Mark Rosewater considered it part of the awesome design for Magic (along with a few other things).

If you want to find out about Magic, you can do worse than read what Mark Rosewater has written.

I think it's because of the large amount of different possible decks, all with different playing styles. It takes players a lot of buying different cards and trying different deck combinations to find what they like the best.