The primary reason is maximizing storage. Because the wheels don't use up valuable space to comply with carry-on regulation, there is more usable volume. This is especially important for maximizing storage for your garments, clothing, and items. The Garment Luggage has an industry-best of 45L of volume. 

The secondary reason is durability. More things can go wrong with four wheels than two, and wear and tear on wheels was a serious issue with us for a frequent traveler. Since spinner wheels stick out and spin in different directions, they are more vulnerable to breakage.