>>21412210It's because peanuts aren't technically nuts the that almonds, hazelnuts, pistachios etc are which are typically the seeds of fruit that grows on trees. Peanuts as the name implies are a species of legume like peas and beans, so they are far, far easier to grow in large amounts. Because they taste nutty and most people like them it's become the go-to form of nut for most cheap confections.
>>21412214 has a point that if you go to ethnic foods, peanuts are much less common.