I'm working on a high-fantasy setting with the ANSI standard complement of fantasy races. I'm using a map of Earth, since all the geological work has been done for me. Here, the dwarves led a bronze-age civilization in the eastern part of North America, which had been destroyed by natural disaster. They fled literally into the Appalachians, and have pushed back out slowly, expanding with trade over generations and extensively colonizing the Plateau and Valley-and-Ridge provinces of the Appalachian range. The site of Pittsburgh in particular has become the gateway to dwarven lands, with a large fortress dominating the river confluence there, right at the site of the historical Fort Pitt and Fort Duquesne, with dwarves and other races living above- and below-ground in the surrounding area.
Pittsburgh is about 35 mi (60 km) from the nearest mountain ridge (near Mount Pleasant), and 70-80 mi (110-130 km) from the nearest big one (near Berlin or Altoona). While tunneling east would make for a shorter trip, water travel was almost always more efficient in medieval times, and one of the rivers that the fortress would control, the Monongahela, flows south into the same mountain ridges via a tributary (near Morgantown, WV for the first; and near Kingwood, WV for the second).
What would make the dwarves prefer to tunnel to their heartland in the mountain halls, rather than float to it, assuming that these older, "interior" mountain halls themselves are already connected by tunnels?