What is the medical reason a person cant recover 100 percent?

The brain does not regrown neurons. The number of neurons (brain cells) stabilizes by 25 years old by most estimations. In the rest of the body, cells have very narrow and specific tasks, like muscle, blood vessel, etc. The brain has over 4000 different "classes" of neurons. Each class have a myriad of specialties. It takes a long time for it to repair axon and dendrite connections.

If you look at growing up as a 20 + year process in brain development and maturing, it is easier to understand how damaged neurons would have to repeat this 20 year cycle to develop and mature. There is such a need for proper function in neurons that when one is damaged, after a while of it struggling to recover unsuccessfully, those surrounding cells cause it to die without replacement.

This does not mean that neuroplasticity and other recovery processes can't help the injured party regain much of the lost functions. Under developed skills can be improved to enhance ability to work-around and accommodate lost functions. Senses can modify greatly to increase a specific sense.