Why there is General Condition of Contract GCC not to be changed or adjusted and there is Special Condition of Contract SCC ?

The GCC contains standard terms and conditions which are generally applicable for all contracts irrespective of the nature of work, supplier type and other factors. Whereas the Special Conditions of Contract or SCC relates to the specific contract. It includes terms and conditions which are applicable for that particular contract only and will vary for each contract depending upon the nature of work, supplier type and other factors. 

It would not be advisable to change or adjust GCC as these terms and conditions might have a long term legal implication and therefore need to be veted by the legal team. It is more of strategic in nature. Special Conditions of Contract will mostly consist of clauses which are applicable only to the particular job or contract. Although it is important to get the SCC veted by legal also but since it is generally specific to one contract and more of tactical (operational) nature, it's implications may not be as deep as the GCC.

To simplify the answer, it is like if you want to change the look of your home for a particular event or function, you generally do not tamper with the foundation of the building but may change the interior decoration for different events.