I’m sorry but basically it’s just that. It has expanded from the first few followers who were Jewish into what we have today. Just because GM produces many different cars they’re still GM cars ... when Christendom began to become more and more political than religious the persecutions of the Jews became more intense and basically divorced itself from the original .... what would you call Jews who believe in Jesus and formed their own group within Judaism?
If you want to better understand the differences, please read Romans or Hebrews, written in the First Century AD.