why did the two men have such a hard time when they tried to harness Buck to the sled?

The two men have difficulty harnessing Buck to the sled because Buck has decided that although he now understands submission to a degree, he is also not willing to accept anything but the lead position in the pack. His growling, snarling, and bucking of the harness pay off as Francois does give him a place in the front of the group.