Why were the dogs unable to move the sled after Hal and Charles packed the sled?

The dogs were unable to move the sled because the runners were frozen in the snow.

"It's not that I care a whoop what becomes of you, but for the dogs' sakes I just want to tell you, you can help them a mighty lot by
breaking out that sled. The runners are froze fast. Throw your weight against the gee-pole, right and left, and break it out."