There are many local reasons for outdoor or yard representations of Mary, but there are other more generic explanations.
Catholic religion is a very visual religion.
Influenced by Eastern Christianity, Catholic piety is iconic, that is, it uses images to bring the divine close to the human and to lead us humans by way of holy images to the source of holiness. Images are not an end in themselves, they have mediating character. Just as Mary has always been considered our main mediator with her Son. Many old and venerable Marian images are considered miraculous and were found outdoors in fields, trees, rivers and caves indicating thus that their origin was not of human doings and dealings. No doubt that there was also a connection between the generative forces of nature and the life-giving function of the mother.