Answer:
Geographical isolation and different goals.
Step-by-step 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.