Because the American Colonies did not grow…

.. or process much cotton until about 1800, people who had money imported silks or cotton from India, carried by English ships.

The special worm that spun silk would not grow on North America.  India had that special silk worm in a special area that made them the only source of that kind of silk in the world.

18th century silk fabric from India was much thicker and lighter in weight than anything we have today.  It made the shape of the gowns stiffer so they could be very big but not too heavy.  The 1770’s silks made a special sound when a woman walked in her skirts that was notable of the Era.

(Photos from today:  silk cocoons in Uzbekistan and silk sari from India)