In a word: flexibility. It supports an identical API to SC, plus: `sx` prop; 0-config SSR (albeit w caveats); a history of superior performance... that's off the top of my head. Also [and with disclaimer that I'm not a huge fan of Material-UI], MUI's maintainers examined their options for their big v5 rewrite, and chose Emotion as the foundational styling engine of choice, aligning with other modern libs like Chakra and theme-ui.