Sharpey's fibers are primarily composed of collagen because collagen provides the necessary tensile strength and durability to securely anchor the periosteum to the underlying bone. This robust attachment is critical for maintaining the integrity of bone structure during mechanical stress and surgical manipulation. Unlike elastin or keratin, collagen's fibrillar structure enables it to resist stretching, which is essential in surgical procedures involving bone periosteum elevation or fixation. Understanding that Sharpey's fibers are mainly collagen helps surgeons anticipate the tissue's behavior during dissection or repair.

Reference: Gray's Anatomy, Susan Standring, 42nd Edition.