Why unity use DX11 by default instead of DX12?

You can use DX12 in unity like this: Go to the Project Settings/Player/Other and uncheck "Auto Graphics API for windows". Now a list of graphics APIs appears. There you can add DX12 and move it to the top.

As you can see, the DX12 implementation in unity is still experimental. That is probably the reason that DX12 isn't the standard graphics API in unity. Another reason could be that the current DX12 implemenation seems to be way less performant than DX11