[Ducky]

Cards can become banned/illegal for a number of reasons. Generally they're only illegal in certain formats (game types). a card could be banned in the tournament formats but still be playable in standard or other formats.

Generally they're banned when the rules text can be interpreted in a way that they had not intended that leads the card to be broken in some way, or when they change the mechanics of something. like a newer card that has the same effect as a old one, but for more mana. the cheaper old one will become illegal.
then there's cards that simply turn out to be more powerful than they had intended after release, so they have to make them illegal to make things fair.