Material, overall design, etc.
E.g., Deep Vs are a little heavy, but they are a little more aero/durable also. New box section rims are usually lighter (less material), but they are less aero. CF is lighter than aluminum, but high end ones have very deep sections which push the weight up some. Hubs also will vary in weight. Trispoke, or disc wheels are obviously the most aero options, and are relatively light because they are usually CF, but the extra material adds more weight than would be there otherwise. For your purposes (it doesn't sound like performance is your top concern), I would look for something that can withstand street riding. Keep maintenance low. The variation of wheel weight isn't a ton at your price point, and it will only be noticeable when you're riding.