Because a VM requires you to manage a window for the VM itself. WSL is more seamless. Speaking of seamless, VirtualBox has a "seamless mode", where guest OS apps can appear as normal host OS windows. However, if you're running a "full Linux distro" inside your VM, this isn't very useful, because you'll have this ugly and tricky-to-manage Linux GUI widget also (the dock on GNOME, or the taskbar on KDE). WSL also lets you launch Linux apps from the Windows start menu, which you can absolutely achieve with VirtualBox, but it'll be a manual effort of writing BATCH files.