Why is the introduction of genetically engineering lymphocytes into a ADA deficiency patient not a permanent cure ?

Adenosine deaminase (ADA) deficiency is associated with Severe Combined Immuno Deficiency (SCID). 

(a) ADA is very crucial for the immune system to function. The deficiency of ADA causes severe combined immuno deficiency disease. The patient Iack functional T-lymphocytes and fails to fight the infectious pathogens. 

(b) Using gene therapy, lymphocytes are extracted from the patient's bone marrow and a normal functional gene for ADA is introduced into these Iymphocytes with the help of the retrovirus. 

(c) The lymphocytes of bone marrow contain the functional ADA gene and reactivate the patient's immune system. 

(d) In some children, ADA deficiency can be cured by bone marrow transplantation, in others it can be treated by enzyme replacement therapy, in which functional ADA is given to the patient by injection. But the problem is that they are not completely curable. If the gene isolated from marrow cells producing ADA is introduced into cells at early embryonic stages, it could be a permanent cure.