Astro has "server islands" which rely on a backend server running somewhere. If 90% of the page is static but you need some interactivity for the remaining 10%, then Astro is a good fit, as that's what makes it different than other purely static site generators. Unlike Next.js, it's also not tied to React but framework-agnostic.

Anyways, that's why it's a good fit for Cloudflare: that backend needs to be run somewhere and Astro is big enough to have some sort of a userbase behind them that Cloudflare can advertise its service to. Think of it more as a targeted ad than a real acquisition because they're super interested in the technology behind it. If that were the case, they could've just forked it instead of acquiring it.

From Astro's perspective, they're (presumably) getting more money than they ever did working on a completely open source tool with zero paywalls, so it's a win-win for both sides that Cloudflare couldn't get from their vibe-coded project nobody's using at the moment.