If a person with bipolar disorder is advised not to meditate or gymming. Why would it be? Does the person face difficulty in concentrating?

Puzzling. Usually good diet, exercise and mindfulness are helpful for mood disorders. Perhaps you're one of those folks who such things to excess and your doctor is trying to help you get some balance in your life. Time for you to ask this question directly to your doctor.
Treat BP first. Untreated bipolar disorder, depending on the mood phase that you are in can lead to excess in both activity (if manic) as well as negative preoccupations (if depressed). If you are stable and on your medications consistently, I do not see a problem with participation in meditation or exercise. In fact, both could be beneficial.