Why were the dogs sold in Skaguay?

Francois and Perrault had done their mail duty. THey didn't need the dogs anymore.

Months of constant toil have depleted strength and dissolved their muscle fibers. The men were tired as well and confidently told the dogs they would be getting a long rest. But, so many men had arrived at the Klondike that the men immediately received new orders, and were told to sell of their dogs and replace them with fresh ones.