Zircon

Another reason for not using the ideal cut round shape for all diamonds is simply that the ideal cut is not ideal for all people and all occasions. The ideal cut emphasizes particular aspects of the diamond, such as the brilliance and fire. However, these characteristics are primarily brought out in bright, modern lighting. The more romantic lighting of candlelight and gaslight is more complementary to older diamond cuts, such as the rose cut, and the step cuts, such as the emerald cut. While round diamonds, ideal cut or not, enjoy a high level of popularity diamonds themselves are as unique as the people who own them. Even though the round ideal cut is a popular choice, it is by no means the standard.
In short, not all diamonds are ideal cut diamonds because not all can be, not all are monetarily able to be, and not everyone wants them.