The Theory is that they didn’t have any animals that could pull a wagon or cart. The horse was native to North America but didn’t survive the ice age.
The bigger question is why they didn’t develop metal tools and weapons. At least in meso-america and S. American they had some pretty advanced skills,but mostly used them for artwork.
In the case of Meso-America, bountiful sources of obsidian negated the necessity of metals for tools and weaponry. Obsidian takes, and holds a better edge than either copper, or bronze, so these were not developed by the local peoples.
In the case of South America, there was some usage of copper in weaponry prior to the arrival of the Spanish, and had they not arrived when they did, bronze, and perhaps even iron would have eventually been utilized. We will never know for certain, though, as the natural progression was interrupted.
The civilizations of Meso and South America during the 1500's AD was closely equivalent to Sumer, and early Egypt, and had they reamined isolated would have progressed, and expanded. The nomadic, and semi-nomadic tribal cultures of North America were essentially doomed any way you care to look at it. If it were not the Europeans, it would have been the Islamics, or Asians, or even the civilizations to their south which would have displaced them, simply because they could not effectively hold the resources they posessed with the manpower and technology available to them...
the infowarrior
Actually, the horse, the camel and a whole bunch of other large (and a few small) mammals survived the (most recent) ice age and all went extinct in a very short period (possibly only a century or two) after the ice age ended.
A popular theory for why this occurred is that this is the period when humans first entered the New World and we wiped all these species out.
Unfortunately for the cool theory, there is increasing evidence men had been banging around America for thousands of years at this point. Possibly ten thousand or more. It also doesn't explain how horses, camels and other animals that went extinct after the ice age in America survived just fine in Eurasia, where they'd been hunted for much longer.
OTOH, we have excellent evidence of similar extinctions when humans first arrived in various Pacific islands, New Zealand, Madagascar and possibly Australia.