Because rats will keep at it for longer. These particular rats are probably bigger than some tiny dogs, but a rat is a forager, so it can be motivated to keep this up for hours. This behaviour is not so close to what a dog needs to do to eat, so it would do it to please but lose motivation eventually.
Well that's what I remember reading some years ago anyway. I guess a dog would keep going if you made a fuss of him, but you don't want to go over to the middle of the minefield to do that.
A previous article said they only work the rats for 30 minutes a day.
Small mammals typically don't have a lot of endurance. Dogs and humans are kind of outliers there, as we specifically evolved for persistence hunting.
It's probably just due to mass and ease of training.
But dogs are trained to sniff truffles and definitely stick to it for long, and the process (sniff around, signal to handler, get reward, repeat) seems the same.
Well, that's just what I read...
I'm no longer sure it is accurate. Looking at their website APOPO now actually use dogs as well. They say that rats and dogs are complementary, but they give an advantage of the dogs but not of the rats
Also, I ran across this article criticising the rats effectiveness: https://nolandmines.com/APOPO%20rats.html . I have no way to assess it's accuracy.