Why is soba not entirely buckwheat flour? I thought that soba was completely made of buckwheat flour, but I learned that it's actually around 80% buckwheat and 20% wheat flour. 

Is there a reason for this mixture ? Is it for flavour or ease of preparation or cost ?
Buckwheat flour is gluten free.  The small amount of wheat flour gives it more stretch, and helps it stay together a bit better.  For more information, check out this article.