Probably because this is obviously serum derived and contains low levels of cross-reacting antigens whether these be bovine cytokines or other antigens. You could try the Pierce protein-free blocking agent which has overcome this problem in tricky assays that we have developed, available through Thermo Fisher.

You may test this potential contamination by using only 0.1% of BSA. The "specific" background should be lower.

Please check carefully for contamination and for the manufacturer's instructions.