I've get through bone marrow transplant about 1 and a half year ago. but why my body still get sick easily and my immune system getting worst?

Bone marrow: It is a tough procedure to receive. Your body is still recovering from all therapy you received in the past. Perhaps your WBC are still low too- that makes you prone to have infection. If you had an allogeneic transplant and are taking medication for gvhd for instance- that can also affect your white blood cell and makes you immunocompromised. I recommend you to f/u with your transplant team.