Because shear forces are ignored in the basic analysis of plane truss elements. Structural models that take into account shear stress (Timoshenko beam theory) will give results involving the shear modulus, which is a function of Poisson's ratio.
Just as an aside LINK elements are typically not used for trusses. For modeling a truss, you would really want to use a BEAM element. You use LINK when either trying to simply tie together a couple of different parts/regions or need tension/compression only spars.