This is the genetically predetermind maximum length hair follicles are capable of growing as a result of the active growth cycle of the strands
Hair grows in predetermined cycles. The maximum length hair is capable of growing is determined by the maount of time it passes through each of the growth cycles before it transitions to the resting and then ultimately the shedding phase
The longer the growth cycle, the longer hair can grow before being naturally shed from the roots
As of current times there is no way to alter maximum terminal length based upon your genetics.
Some people, due to the mishandling and mismanagement of their hair, mistakenly believe they have reached their maximum hair length when they haven't.
The reality? Hair has become so damaged from a variety of reasons that it is too damaged to conitnue to grow. Thus is doesn't reach it's maximum terminal length due to damage and not because of other factors
This is a common issue which is also easily resolved issue once ongoing hair damage is reversed or healed.