I wonder, too, and would like to know. I figure it has to do with the house seeming so large to them, like Linda said, that the rest of the house seems like the outdoors. Or, the carpet just soaks up their small potty so they don't smell it enough to register?
It don't believe it has anything to do with bladder size, because their bladder is in proportion to their body size compared to any other dog, as is their water consumption.
I wonder if there is some gene that is different in Havs and another couple of small breeds... (although I'd box anyone's ears that tried to say my Tucker was not smart!
Whatever the reason, I do wish someone would figure it out and fix it! Ha! I got Tucker knowing housebreaking would take a long time of intensive effort, (which it did!) None of my bigger dogs took long at all to housebreak, with a whole lot less diligence required on my part.
But, if I ever get another dog, it will still be another Hav!
ETA: I also will look for a breeder that does start the house training process from the youngest age! I do know that the breeder I got Tucker from just let the pups go on paper where they wanted to in their ex-pen. Maybe with the small breeds that makes all the difference. (My larger dogs, however, didn't have special training started by their breeders and they were easy to train.)