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Hills need a focus and the trees are good. Keeping other colours out of the image forces the eye to be more sensitive to the shades of green. This way, the viewer appreciates the fine differences. As all the greens are natural, the eye is satisfied. The image works. Taller grass gives a variety of greens that adds interest. Since green is the most restful of colours, the image calms the viewer - it is a visual escape from stress. That makes it immediately appealing. More than that, the image is sensitively selected: there is balance in the image - nothing is either too central or too marginal. Overall, you use the gentlest of colours to give the viewer an instant experience of tranquility.