Spain has a lot of remote rural areas. Must likely these stones were overgrown in tall grass, bushes or covered in dirt.

I walk a lot in remote areas but even there it's very rare for people to leave the trails.

And finally it's possible that some locals were aware of some "rocks in that field" but weren't aware of the archeological significance.

Be aware that the photo in the article is not from the site in question but one in France that's much bigger and maintained.