^^^ Yup, that is a big part of it.
Males are generally larger, stronger and more aggressive. Males do not go into heat and you don't have to deal with a female that may become flaky during her heat cycle. Don't get me wrong, there are some strong females out there. But, I would prefer a male for a PP dog.
Yes, most Patrol K-9's are male. For pretty much the reasons stated above; size, aggression and strength. I rarely see females for sale when testing and selecting dogs for patrol work. A really good, strong female is worth much more to a breeder than a one time sale. They keep those dogs back for breedings and make more money. That is another reason that we do not see a lot of females offered for Patrol work. You don't see the goose that is laying the golden eggs.
“Beware of false knowledge; it is more dangerous than ignorance”. George Bernard Shaw