The truth is a bit of everything, bun being a messy codebase written primarily with "move fast and break things" in mind, cultural divergence between Bun and the Zig community, and also hiring issues. People maybe forgot but Jarred at some point caused a bunch of drama when he tweeted that working at Bun is not for people that value life/work balance, which went viral and caused an uproar. Must not have been super easy to hire from the Zig community after that, and in fact once Bun got acquired by Anthropic, it was pretty much Jarred and Claude doing all the work on the codebase. Pivoting to Rust is probably at least in part a way to reset the clock on those hiring interactions.