"The sky is high" in Ilocano would be "Naindak ti langit."
However, the question "Why is the sky high?" doesn't quite make sense in English. It's important to understand that the Ilocano phrase you provided is a statement, not a question.
To make a question about the sky in English, you could ask:
* Why is the sky blue?
* How high is the sky?
* Why does the sky seem so vast?
Please clarify what you'd like to know about the sky, and I'll be happy to help!