When the blood of theRh-negative mother comes in contact with the blood of Rh positive foetus it starts forming antibodies against the foetal blood.Why then the foetal blood doesn't any antibodies against Rh negative blood of the mother???

Antibodies are produced against an antigen, rhesus factor of the foetus acts as an antigen and mothers body produces antibodies against it. Rhesus factor negative means that there is no rhesus factor present hence no antibody is produced, this is the reason for foetus not producing antibodies against mothers blood.