Because that would be a much shorter series.
This is the story of the idea of sparing animals from the dinner table; the forms that idea took, and the people who passed it on. Through the vast majority of history, that idea went as far as lacto-vegetarianism. So titling the full story "Vegetarianism" rather than "Veganism" is simply historically accurate.
Donald Watson, who founded The Vegan Society in 1944, called veganism "the beginning and the end of vegetarianism"; even as he was making history, he put veganism in the historical context of vegetarianism.
You can read more about how "The Vegan Option" combines animal ethics with journalism at: