Lot of commuters feel MTN bikes are more versatile, are like the bikes they grew up with, more maneuverable, can jump curbs when needed, easier to mount and balance or lay down and bail, and basically feel they are more rugged and can take more abuse than a road bike. Parts and consumable items (tires, schrader tubes, chains, brake pads, etc) can be bought/found almost anywhere (drug stores, "x"-marts, corner hardware store, etc) and don't cost an arm or a leg or need an adapter to fill up your tires at a service station. A few prefer disc brakes on their bikes without having to pay $1500.00+ to get a Trek Portland, Cannondale Cyclocross Disc, LeMond Poprad, etc.

The classic 3-speed is making a comeback of sorts...updated with 700C wheels and marketed as an urban/town/commuter bike. Schwinn's Coffee and Cream is the latest in this category and comes with fenders and a rack. I expect to see a few of these on the road as a faster alternative to a MTN bike, esp if the commute is relatively flat. Would have been better if 7-8 speed hubs were used.

But people/commuters make decisions that best fit their needs and requirements. There's no real right or wrong in commuting bike choices. MTN bikes just sell better and there are more choices at prices people are willing or can afford to pay and a lot feel offer more value for the money.