More than likely there were several young Arab males travelling on these flights whose backgrounds could not be checked, and it was not politically correct to single out these passengers, take them off the flight, and let the flight proceed with the remaining passengers.
Singling out particular passengers because they "looked suspicious" resulted in multimillion dollar lawsuits here in the US against some airlines in the past few years. As a result, it is now more politically correct to cancel the entire flight and ascribe the excuse of "security concerns" to the cancellation rather than risk the accusation of "racial profiling" and lawsuits in singling out one or two passengers.