The most likely reason for the breeding of the tomato plants. When one mixes plants together, they have to be sure they are compatible. Otherwise, this could result in failed plants and perhaps even a new type of tomato plant which takes over other plants. Because of this, breeders have to consider every detail when creating a new type of tomato. Another reason is time. In most casinarios when you breed two plants together, you have to take a part from each plant. It could take months for the plants to breed, and they may not even end up being successful!
