I am being sincere and a little self deprecating when I say: because I prefer Gen X-coded projects (Node, and Deno for that matter) to Gen Z-coded projects (Bun).

Bun being VC-backed allows me to fig-leaf that emotional preference with a rational facade.

Because Bun is still far less mature (and the stack on which it builds is even less so - Zig isn't even 1.0).

Because its Node.js compat isn't perfect, and so if you're running on Node in prod for whatever reason (e.g. because it's an Electron app), you might want to use the same thing in dev to avoid "why doesn't it work??" head scratches.

Because Bun doesn't have as good IDE integration as Node does.

I haven't used it for a few months but in my experience, its package/monorepo management features suck compared to pnpm (dependencies leak between monorepo packages, the command line is buggy, etc), bun --bun is stupid, build scripts for packages routinely blow up since they use node so i end up needing to have both node and bun present for installs to work, packages routinely crash because they're not bun-compatible, most of the useful optimizations are making it into Node anyway, and installing ramda or whatever takes 2 seconds and I trust it so all of Bun's random helper libraries are of marginal utility.

Why bother with bun when deno 2 is a much better alternative for new projects?

because bun is written in a language that isn't even stable (zig) and uses webkit. None of the developer niceties will cover that up. I also don't know if they'll be able to monetize, which means it might die if funding dries up.