Why did the Torah require the wearing of the blue threads?
Because the blue resembles sapphire, and the Tablets were of sapphire, so to gaze upon the blue threads would be a reminder of that which is inscribed on the Tablets and a reminder to fulfill what is written there.
The Phoenicians were the ancient sea merchants of the Middle East and the traders of blue dye, which was centered on the Mediterranean coast and famous throughout the ancient world. This blue dye was derived from snails and was so rare and sought after that it was worth its weight in gold. It colored the robes of the kings and princes of Media, Babylon, Egypt, Greece and Rome. To wear it was to be identified with royalty. Thus the blue thread was also a constant and conspicuous reminder of the stature of Jews as noble sons of the King of the Universe.
Blue dye production slowly came under imperial control. The Romans issued edicts that only royalty could wear garments colored with these dyes, and only imperial dye houses were permitted to manufacture it.
Because the blue dye became problematic, the commandment to have a thread of blue in each corner of the prayer shawl was waived in the second century of the common era. With time the secret of producing the dye was also lost.
Recently there has been a revival of interest in the blue dye in Israel. A member of the Israel Fiber Institute has published a number of articles on the subject, and a professor of the Shenkar College of Fibers has carried out chemical analysis of the dye from present day snails as compared with samples from archeological artifacts dating back 3,200 years.
As a result of these and other efforts the threads are now again being produced in Israel.