Reasons for Lack of Flexibility

Muscle imbalance.

Many times the agonist muscules are far weaker than corresponding antagonist muscles on opposite side of joint.
Overuse of the muscle without maintaining complete range of motion. Activities that are constantly performed without stretching will result in tight joints and muscles. Muscles such as the gastrocnemius, rectus femories, iliopsoas and hamstrings shorten from continuous stress of activities such as running, jumping, stopping and starting.
Periods of rapid growth. Body coordination is affected during times of growth when the body displays rapid change in height or weight. This is illustrated by a temporary loss of coordination as musculotendinous structure flexibiliy fails to keep up to the increased long bone or soft tissue growth.