Sad Structure

As humankind develops its spacefaring technology, so grows the hunger for exploration in space. Recently, just a few years after the beginning of the New Terraforming Era, humankind has stumbled upon the first evidence of the existence of life outside earth and its colonies.
While constructing platforms for housing on a suitable planet, a temple-like structure was uncovered. Amongst a lot of lowtech cultural findings, the humans also found a sculpture depicting a god of the race that lived on that planet.
The temple and its god are now nothing more than an exhibition. Although this temple is a cultural treasure, it offers little answer as to what happened to those who lived on the planet and why that temple is the only alien structure on the whole planet.