This paper from 2010 has some info on why this may be the case [0]: "These results indicate that the protection of bacteria on the leaf surface by biofilm forma

This paper from 2010 has some info on why this may be the case [0]: "These results indicate that the protection of bacteria on the leaf surface by biofilm formation and stomatal colonization can reduce the antimicrobial efficacy of irradiation on leafy green vegetables." [0] https://www.ncbi.nlm.nih.gov/pubmed/20629883 https://www.ncbi.nlm.nih.gov/pubmed/20629883