Why are there different branches and schools of thought on Buddhism?

Geographical isolation and different goals.

Explanation:

Theravada Buddhism is more stripped down and bare than Mahayana Buddhism, and the simple reason is that Mahayana Buddhism was practiced in more prosperous areas (coincidentally near the Silk Road at the time), so merchants would pay good money or give expensive goods to Buddhist monasteries to ensure good business or safe travels. The more stripped-down Buddhism is nearer to the birthplace of the religion, India, where Theravada Buddhism was the minor religion and thus became more of a spiritual practice with less rituals. Hirayana... yeah you're on your own. Sorry Hirayana Buddhism doesn't really have a reason and it's pretty obscure to even see in practice in 2020.

they were unable to pay their debts

the east coast has nothing to do with the dust bowl, the dust bowl killed a full year's worth of crops in seconds, and with the dust on the land, the land would be worthless.   furthermore the farmers couldn't  get  out of their houses due to the dust so when the bank came to collect money, farmers couldn't  pay it

d is the answer. leo should include the source.