Why IV in vein not artery?

Because it is easier, safer, and less painful to puncture a vein than an artery. The vein is larger than the artery and has a thinner wall. The artery is covered in a muscular layer, while the vein is not. The angle that you would have to use to safely enter an artery would be awkward if sitting across from a patient, while the angle of entry for a vein is straight on.