I'm sitting in an airport, so I don't have my D1.1 handy.  However, in response to "why GMAW cannot be used in all position," my response is because the presumption is that the GMAW is in the globular or spray transfer mode if you are using a prequalified WPS. Neither welding process can be used in vertical or overhead positions with the prequalified ( spray or globular) transfer modes.

You can use the short circuiting transfer mode for all position, but the process is not prequalified.

D1.1, like many other codes, will let you do anything you want in production if you can demonstrate you can apply the process and meet the code requirements by qualifying the WPS per clause 4.

Best regards - Al

I believe the reason for not listing the Horizontal position for the B-U2a is that you would normally be using a different joint configuration for the Horizontal postion, something like a B-U4a for instance that gives the welder a shelf to build his fill passes on.