I haven't used Sapper, but I've read a bit about SvelteKit. Afaik the main reasons for the rewrite were to clean up the code, and make the framework platform agnostic. This means that you can use different adapters for SvelteKit to make it usable in places where Sapper wasn't optimal, such as serverless hosts.
For more in depth info, refer to this page: https://kit.svelte.dev/migrating