Translation:Why are you so quiet today?
Both your and given translations are correct, but are used in different situations and at certain type of expressions.
那么 literally means "so". If you were to say 为什么你今天那么安静, your sentence would express close to the following
"Why are you so quiet?!"
where "so" increases the extent of "quiet". Likewise, since 怎么 can be used in why/how-come sentence, the sentence serves the same purpose.
Thus, 为什么你今天很安静？ serves the best meaning at a neutral emphasis.