Because it takes a lot more work to appear blue: 

For a plant to appear blue, it needs to be able to produce a large and complex molecule that can absorb very small amounts of energy due to the small wavelength of blue light. In the few blue-colored plants, the blue color is generated by blue pigment, namely anthocyanins. 

In animals, blue coloring is generated through structural optic light effects, and not through colored pigments. However, as with plants, only a few animals appear blue due to the structural complexity required to generate said light effects.
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