Why is the sentence structure Pourquoi ce paquet que tu portes est-il si léger? and not Pourquoi est-ce paquet que tu portes si léger ?

See a translation

"Pourquoi ce paquet que tu portes est-il si léger ?" (Subject-verb inversion, "est-il" (is it) instead of "il est" (it is))
"Pourquoi est-ce QUE CE paquet que tu portes EST si léger ?" is a correct sentence. There is no s-v inversion because of the interrogative particle "est-ce que". Since it shows that the sentence is a question, there is no need to reverse the subject and the verb at the end.

Literally, it looks like this "Why, this package that you carry, is so light?" (for the 1st sentence) and "Why is that this package that you carry is so light?" (for the 2nd). The 1st one, in French, is the most polite form to ask questions (with the s-v inversion) while the 2nd is just neutral (with the "est-ce que" particle and no s-v inversion). Both are correct and can be interchanged.