The answer to this question can be found in Chapter 5. It's actually one of my favorite sequences in the book because it's slightly humorous how ignorant Mercedes and the two men are about the conditions, the dogs, the sled, and necessary supplies. The men from the area find it funny as well and wink at each other at a shared inside joke.

Three men from a neighboring tent came out and looked on, grinning and winking at one another.

“You’ve got a right smart load as it is,” said one of them; “and it’s not me should tell you your business, but I wouldn’t tote that tent along if I was you.”

As the quote indicates, the knowledgeable men from the area try to gently hint at the fact that the load is too large for the dog team. This is the first reason that the dogs can't get the sled moving. A second reason is that the dogs haven't been given adequate enough time to rest from their previous work. Again, one of the experienced men points this out to Mercedes, Hal, and Charles:

“They’re weak as water, if you want to know,” came the reply from one of the men. “Plum tuckered out, that’s what’s the matter. They need a rest.”

Unfortunately, those three are too stubborn and too impatient to wait longer. The final reason that the sled won't budge is because the sled's runners have become frozen solid to the ground. Again, one of the locals has to point out this fact to the ignorant party. His comment is great too. He flat out states he doesn't care what happens to the people, but he does care about the well-being of the dogs.

“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.”

The answer to your question takes place in Chapter 5 of Jack London’s The Call of the Wild. The sled was too heavy, due to Hal and Charles overloading it with equipment, including a tent that they wouldn’t really need because springtime was coming. The dogs were tired, which was pointed out to Hal and Charles by an onlooker, but they ignored him, whipping the dogs to try to force them to move. Finally, the runners were frozen in the ground. Hal broke the runners out of the ground but once freed, the sled tipped over and spilled half of its contents. The dogs, angry at the treatment they’d just received, broke into a run and kept going until they were caught by people and returned to Hal and Charles.

The dogs were tired, the sled was overloaded and too heavy, and the runners were frozen to the ground, this made it impossible for the dogs to start the sled forward.