Why are tendons made of dense regular connective tissue but the dermis contains dense irregular connective tissue?

Different from dense irregular connective tissue, dense regular connective tissue (CT) is packed with collagen, a strong fibrous protein, but in a different way: the collagen fibers in the dense regular CT are arranged in parallel lines, whereas the collagen fibers in the dense irregular CT are packed in all directions.

The more ordered the tissue, the more robust it is; the reticular layer of the dermis (where dense irregular CT is found) does not require the same level of organization of collagen fibers as the tendons and ligaments, which must withstand all of the stress we place on them, including bearing a large portion of our body weight.

Tendons require strength and unidirectional force transmission, hence dense regular connective tissue. The dermis needs to withstand multidirectional forces, leading to the presence of dense irregular connective tissue.