Re: Probable cause
Oddly it falls down to the ease of training from what I remember from chatting to the MoD dog teams back in my uniform days.
Basically you need a "smart" breed, and that tends to be the gun-dog type rather than the sniffers/pack hunters. That gives you the basis for teaching them and having them get the concept.
Then you need a strong "prey/play" drive, which is reward focused and allows conditioning for behaviour and task. As mentioned in the article, "I find the thing, I get my toy".
Finally it is a question of multi-role, something like a K9 team being deployed by police and military is more useful if the dog is big enough and trained to also do suspect tracking and pursuit. Think shepherd breeds etc, they are fast, focussed, and you sure as hell go to the floor when one does an arm-bite takedown on you. Less so with a beagle...
In fairness, there are things like spaniels on some police sniffer dog teams, but when on patrol they are often deployed with a German shepherd in the car too. (The big estates they deploy in hold a couple of "kennels".