Why are tendons made of dense regular connective tissue but the dermis contains dense irregular connective tissue?
Tendons and ligaments are made of dense regular connective tissue because they need a strong structure.
The dense regular connective tissue (CT) is different than dense irregular connective tissue. Both tissues are densely packed with collagen, a strong fibrous protein. However, they are organized differently. The dense regular CT's collagen fibers are arranged in parallel lines where the dense irregular CT's collagen fibers are packed in all different directional orientations.
The more organized the tissue, the stronger it is. The tendons and ligaments have to be strong to stand up to all of the stress we put on them, including holding much of our body weight. The reticular layer of the dermis (where you find dense irregular CT) doesn't have to be as strong. Therefore, the collagen fibers don't have to be as organized.