Things like smallish screens getting good and cheap enough, leading to the Oculus Rift and HTC Vive.

Or in the case of the Quest, having powerful and reasonably priced SoC's.

I'd say the breakthrough term was not the ideal word choice - it's more that technological advances passed a threshold:

For VR, a lot was advances in (affordable) processing power (latency causes significant issues in VR), and screen (manufacturing) technology (i.e. pricing improvements).

For self-driving, it's again processing power (with the focus on the ratio between computation power and power consumption/size), but also the advances in technologies like Lidar, as well as advances in theoretical research (and better understanding) of machine learning.

You’re in for a treat if you haven’t heard of the oculus quest :)