I presume they use the material that they do to construct the blades because it works and the cost to make them out of fiberglass makes sense (maybe an assumption). If there was anyway the Green Industry could make them out of something that could be recycled, you know they would. The issue of used blades has been a thorn in the side of the wind industry from the start. If they could market the recycled blades for something useful or recycle the components into something else that was valuable, it would be a big selling point and it would be done.
I can assume that if the Green Industry has yet to come up with some other option for construction or recycling, that there isn't an option, at least with the technology available now.