I think it's very breed specific. Lots of toy dogs are potty pad trained - the smaller dogs often don't like to go out as much and they have smaller bladders (supposedly, don't know how true this is). When I wanted to switch Tari to going outside when she was a puppy, I just moved the potty pad a few feet toward the door every other day, then out the door, then onto the lawn. She figured it out very quickly.
My other 2 do their business 100% outside. I was afraid Tari's potty pads might un-housebreak one of them, but didn't happen.