Why are the vast majority of tallitot made of wool? Why is a silk tallit or a cotton tallit the exception and not the rule? According to halacha, wool is the material of choice for a tallit, although other materials are acceptable as well. The only problem fabrics would be synthetics, according to some opinions, and a linen tallit.
For a very interesting discussion of different tallit materials, including silk and linen, take a look at this thread: http://www.chabad.org/library/article_cdo/aid/1425700/showfeedback/true/jewish/Whats-Wrong-With-a-Silk-Tallit-Prayer-Shawl.htm