Why only 0.1 % peptone is used for sterility testing ( reason besides USP recommend it) ? Why not wfi or any other diluent solution can be used ?
Any sterile isotonic diluent can be used, provided that the nature of the diluent does not inhibit growth and your suitability test results are within specification.
0.1% peptone is known to not inhibit growth, that’s why pharmacopeias refer to this option. Saline is also an option. Water is hypotonic so is likely to inhibit growth of micro organisms found in pharmaceuticals, so not an option.