There's a good write-up of the shortcomings that the new config file is trying to address here: https://eslint.org/blog/2022/08/new-config-system-part-2/

E.g., faster and simpler file access from removing tree-walking for multiple `.eslintrc.*`; better defaults in line with modern JavaScript; better compatibility with modules and Node.js's own file loading.

(And I suspect there's an element of "As long as we're having to introduce a breaking change, let's make it explicitly breaking by also changing the filename.")