They don't have enough funding. Next.js and Gatsby are in an entirely different weight class compared to Sapper.
Rich said it recently in a video. The Svelte team is working on the next big thing combining Svelte with Snowpack. The project is called SvelteKit.
They both seemed like fine pieces of software. They just don't seem offer separate much their counterparts nowadays that warrants continued feature development.
I just watched your presentation at Svelte Summit and I'm looking forward to how Svelte progresses.