Because they were so inferior in every way.
Leghorns mature in half the time. Eat half as much. Need almost half the amount of space. Lay twice as many eggs. Free range twice as well. Can be sexed in weeks instead of months. Are so much more friendly. Etc.
When you want to breed for patterns and breed for several at the same time it's a no brainer.
Leghorns mature in half the time. Eat half as much. Need almost half the amount of space. Lay twice as many eggs. Free range twice as well. Can be sexed in weeks instead of months. Are so much more friendly. Etc.
When you want to breed for patterns and breed for several at the same time it's a no brainer.