Because this is far from the first evidence. There is plenty of evidence for liquid water on Mars both past and present. In fairness, the article mentions this. In spite of the headline, it only claims the first direct evidence. Personally, I think even that is a bit of an overstatement. To me, this is simply one more bit of evidence that supports the current scientific consensus.

This would be the first time they state there's liquid water naturally under one of our machines. For a starter, this is a huge problem regarding environmental protection. Regardless of the other factors against letting something like Conan the Bacterium take residence a few cm under the surface, I suspect if this had been suspected earlier (or those that suspected it were taken seriously), we wouldn't have sent the same kind of probes to the same areas, possibly not even having a Mars ground exploration program at all until we understood better how to do it without polluting the planet with terrestrial life.