Because del is de + el, and you cannot use el in this context. El is the definite article, and the intention isn't to show that the soup was made with an specific tomato (e.g "the big tomato that was on the fridge"). It's just a soup made with tomatoes, it doesn't matter which tomates was used.
Wouldn't "On Fridays" be a much more appropriate answer than "Fridays"? Both were accepted, although the latter sounds significantly less natural - I would certainly never say it like that. Also, "On Fridays" is a preposition, but saying "Fridays" removes the prepositional phrase while still playing the role of a preposition. Are you sure this is right?