"more robust than five years ago" vs. "more robust than it was five years ago"

The truth is, this is not exactly a great question. Both are grammatically acceptable. What you should think to yourself here, however, is which is slightly clearer. The addition of the "it was" verb phrase makes it manifest that the writer is comparing "the construction industry’s economic health" from five year's ago. That said, any fluent English speaker would know that regardless, and the "it was" can be elided.

There are potentially other somewhat similar cases with comparisons where the use of an actual verb phrase significantly boosts clarity, but I would argue this is not one.