Ease of use and ease of setup - both of which are important. If I want to setup an Ubuntu VirtualBox, I have to download the VirtualBox software, find the Ubuntu ISO, and run through the setup process. Then I've got to work through any quirks with the setup to get it working seamlessly within the larger windows environment.

With WSL? I hit the install button on the windows app store. That simplicity is important and vastly increases the immediate utility of something like WSL, especially if you just want to try it out.