Why were they able to travel out faster on the thirty mile river as opposed to going in?

The difference is the condition of the ice on the river, and Buck’s leadership. On the way in, the river is not completely frozen. The men and dogs are constantly breaking through the ice and getting wet, which meant having to stop and build a fire to dry out. It was exhausting and tedious work. Another factor was the conflict between Buck and Spitz, the lead dog. Buck sought to replace Spitz as lead, and to that end sowed conflict and discord amongst the other dogs. This made the team much less efficient.
On the way out, not only is the river frozen solid, but Buck has defeated Spitz and been installed as the new lead dog. Under these favorable conditions, the team is able to cover “in one day going out what had taken them ten days coming in.” Under Buck’s leadership, the team pulled as a single animal, and discipline was restored. As Francois says, “‘Nevaire such a dog as dat Buck! . . . No, nevaire! Heem worth one t'ousan' dollair, by Gar!’”