Actually, VirtualBox's BIOS code is 100% freely redistributable and modifiable. Unfortunately, the only compiler capable of building it is non-Free (OpenWatcom). It turns out there's not a whole lot of interest in writing compilers targeting 8086 real mode these days. :(
1. Qemu supports more architectures, such as running ARM or MIPS code on x86 hardware.
2. Server use. It works with KVM, Xen, Virt-manager, etc; it plays nice with the ecosystem on GNU/Linux. I know my VPS is using Qemu as part of their stack.
3. That last point also means that improvements to KVM and whatnot also improve Qemu.
4. If you are a "freetard", these days VirtualBox uses non-free code in the BIOS. Qemu can be used totally free.
5. As petty as it sounds: It was around first, and people stick with what they know (this is the principle reason I use Qemu).