If the Kingswood Brotherhood was likely a reaction to the policies of Aerys II and Tywin, or at least they grew because of it, why didn’t any groups in the Riverlands or elsewhere form? Local politics? Unsuitable terrain?

Well, keep in mind that the condition of the peasantry, or indeed of any socio-economic group, is never uniform. 

In the case of the Kingswood Brotherhood, you’re dealing with a population of foresters who have to make their living from the King’s own wood. The King’s privileges and exclusions directly impact their livelihood in a way that isn’t the case for many peasants.

From the little evidence we have, the Kingswood Brotherhood used these grievances to rally support from the smallfolk:

“Good luck getting answers then,” said Jaime. “If you want their help, you need to make them love you. That was how Arthur Dayne did it, when we rode against the Kingswood Brotherhood. He paid the smallfolk for the food we ate, brought their grievances to King Aerys, expanded the grazing lands around their villages, even won them the right to fell a certain number of trees each year and take a few of the king’s deer during the autumn. The forest folk had looked to Toyne to defend them, but Ser Arthur did more for them than the Brotherhood could ever hope to do, and won them to our side. After that, the rest was easy.”

Grazing rights, hunting rights, felling rights – these things might seem parochial, but they could mean the difference between starvation and and prosperity to marginal communities like the people of the Kingswood. 

However, if you’re a smallfolk in the Riverlands, you have an entirely different set of interests and concerns, so you don’t necessarily have anything vested in the Kingswood Brotherhood.