This blog post gives a bit more information on that: https://svelte.dev/blog/whats-the-deal-with-sveltekit, though the section on Snowpack is out of date. Basically, SvelteKit is serverless-first (whereas Sapper made you choose between a Node server and prerendered HTML), and because it's powered by Vite has a much nicer developer experience.

It also fixes a few of the rough design edges in Sapper. This migration guide gives you an idea of the scope of the changes: https://kit.svelte.dev/migrating