This is the epicenter of
apparel design and manufacturing in the United States; domestically manufactured clothing is more expensive, but retail and wholesale customers who care about quality and working conditions have historically been willing to pay for it.
Unfortunately, the industry is on a downward trend. Los Angeles County used to have more than 5,000 apparel factories; today, my company is one of roughly 2,000 — and many (e.g. American Apparel) are looking for a way out.