They knew about the wheel (as some of their children toys had them), so the explanation is easy: They did not find it effective. Completely understandable. I spent my youth walking (and carrying a lot) in nature, and not once would I have tried to use a wheeled wagon and definitely not a wheelbarrow during those excursions. If I had a draft animal I would have used it to carry stuff (as people do, with horses and dogs), not a wagon with wheels - that would have been totally impractical. And the terrain where I grew up is way easier, by several orders of magnitude, than what I could see when I visited Yucatan some years ago.