Take taxis, which many economists say will soon be driven by robots. Their numbers tumbled in 2020, but because people travelled less, not because of driverless taxis.

我阅读少，第一次见到这种情况。一般是不用but的，或者用not...but...。原文这样写是为了强调not because of driverless taxis吗？

谢谢老师！