A new report from Research and Markets analyzes how the US denim jeans market fared in 2014.

Jeans are traditionally a strong seller, but in 2014 denim jeans sales suffered a "drastic fall".

Yoga pants are to blame.

Denim has become a wardrobe staple. The report says fit, comfort and price are the factors that customers say are the most important when purchasing denim jeans.

Jeans offer a long life span, which gives the denim industry a strong position over other types of apparel. Jeans may also be worn at both social and work events. The versatility of jeans has long been one of the strengths of its sales.

Jeans sales have proven to be resilient over the years. The global economic downturn severely affected the apparel industry, but the denim market regained sales relatively faster than other apparel markets.

In 2014, the popularity of yoga pants presented jeans with another challenge, a challenge the denim market wouldn't win. Yoga wear's new fabrics, designs and comfort offer a new set of challenges for denim jeans sellers. The growth in sales of yogo pants created a "drastic fall" in denim sales.

The four major companies in the US denim industry, VF Corporation (Wrangler, Lee, etc.), Levi's Strauss , PVH Corporation (Calvin Klein, Tommy Hilfiger, etc.) and Joe's Jeans Inc., all struggled with declining sales of their denim products in 2014.

VF Corporations overall apparel revenues increased by 8 percent in 2014 and profits were up 9 percent, but VF Corporations jeanswear was not able to keep pace. Jeanswear revenues were flat year-to-year while profits were down 3 percent.

It's hard to imagine that the sales of denim jeans will continue to struggle long-term in the United States, but never doubt the power of yoga pants.