Visual Studio 2015 resetting web project to use IIS Express

I edited the .csproj.user file and this fixed the problem. I changed <UseIISExpress>true</UseIISExpress> to <UseIISExpress>false</UseIISExpress> and left everything else the same.

I still don't know why doing this through the UI didn't save, if anyone else can explain it I'll happily accept their answer.

Delete the file projectName.csproj.user. User specified preferences in this file overwrite the project settings.

NibblyPig pointed out in their comment that:

it appears that if you don't change the URL then it doesn't save the setting. So I changed my Project Url, pressed save, then changed it back and pressed save again and it updated the file.

I'm just putting this as an answer so it's easier to find, but full credit to NibbyPig for finding this.