"Flow cytometry was a very rapid and accurate method but it could not be applied to the detection of rare events"
I read this in the following article, but there is no explanation there.
Lemarchand, K., Parthuisot, N., Catala, P., & Lebaron, P. (2001). Comparative assessment of epifluorescence microscopy, flow cytometry and solid-phase cytometry used in the enumeration of specific bacteria in water. Aquatic Microbial Ecology, 25, 301–309. https://doi.org/10.3354/ame025301