There are three popular widths for refrigerators: 36", 33" and 30". Why are they so popular? Appliances fit cabinets. You will never see a 37" refrigerator, because it cannot fit inside a popular cabinet.
That being said, a 36" is by far the most popular and has the most selection. You are probably reading this because you cannot find a decent 30 inch refrigerator. It is pretty amazing, because most city apartments can only fit a 30 inch fridge.
There are very few actually, because most 30 inch seems to be for less expensive ($499 and up) top mounted refrigerators. However, French doors have better refrigeration space and are generally considered nicer looking.
Below are some of the best 30” French door refrigerators.