We unfortunately released Astro 6 only a few weeks before Vite 8 / Rolldown came out, which is why we did Astro 7 so soon. But there are very few breaking changes compared to Astro 6. That being said, some of these performance improvements (the Sätteri processor) are available in Astro 6 too.
Cloudflare bought Astro recently, and as it states in docs it previously had cache plugins for 2 companies but not Cloudflare so that may have been a motivation along with the Vite update mentioned
It was partially that, but mostly the Vite version with the Rolldown bundling etc. We typically always need to do a major whenever Vite releases one because it tends to impact us a lot compared to other frameworks for various reasons.