why zinc shows only +2 oxidation state
Zinc is a member of the 3d–block of elements BUT why isn't zinc a true transition metal Zinc cannot form an ion with an incomplete d sub–shell and is therefore not a true transition element. Zinc's chemistry is determined solely by the formation of compounds in its +2 oxidation state, but it does form many complexes, though not as many as other transition metals.
Zinc has electronic configuration as follows 3d104s2 . So, Zinc is normally not considered a true transition element because of filled d-orbital. It readily loses 2 electrons to form Zn2+ to form 3d10 configuration. So, nomally Zn is found in +2 oxidation state. But at very extrme conditions, Zn can exhibit +3 oxidation state also.