I understand that Wightman axioms were defined for scalar quantum field theories. However, what prevents axiomatization of non-scalar quantum field theories, such as gauge theories, even for free field?
Well, nothing prevents a generalization of Wightman's axioms in case of free gauge theories. This, however, requires in turn a generalization of Hilbert spaces, known as Krein Spaces. The only lucid and thorough presentation I know of is Chapter #10 of BLOT. I hope this boook is available to you.