The explicitly stated reason is that they ran into too many segfaults and it was slowing down development velocity.

That said, I hope that Zig posts won't be plagued by "but the bun thing" like this in the near future.

I am not aure either, but bun wasn't using normal zig and there was drama about upstreaming. Combine that with anthropics desire to show they can help rewrite everything in rust and that probably accounts for some of it.