What is the difference between mountain bikes and road bikes brakes?

Same reason as a Jeep won't do as well as a NASCAR car on a racetrack. The MTB is built heavier and stronger to be able to stand jumps and drops, which the road bike isn't expected to have to hold up to. The MTB riding position gives better view but more wind resistance than the road bike. Road bikes can have real aerodynamic and slim tires, while the MTB need wider more knobbly tires for traction on dirt.

With that said, a MTB w/o suspension but with slick tires can make a real good bike for city/urban riding. Good overview, good brakes, nimble, rugged and manouverable. Usually speeds during urban riding isn't high enough for the improved aerodynamic of the road bike to mean much.

Road bikes tend to be lighter, and faster on downhills.