Artificial Intelligence: theory vs processing power
AI spans many different areas.  If you are actually asking about machine learning, the conventional wisdom is probably that it's a combination of new ideas (e.g., neural networks, architectures, methods for training them), computation power (e.g., GPUs), and data (large labelled datasets).
See, e.g.,
https://qz.com/1307091/the-inside-story-of-how-ai-got-good-enough-to-dominate-silicon-valley/,
https://towardsdatascience.com/the-state-of-ai-in-2020-1f95df336eb0, and probably many more (I didn't spend much time looking).