Because it’s written in C# and relatively new. Unbound is written in C so should consume less resources, has been around longer and has been vetted – FreeBSD and OpenBSD replaced BIND with Unbound.

The one downside to Unbound is that there’s no GUI so it can be a bit intimidating to set up. But the docs are excellent and Unbound defaults are secure, so it’s not as hard as it seems.