Even with a 10-foot-long tank that only 13" wide, larger fish will look a bit silly in a smaller tank because they'd be restricted to mainly side to side movement. With a greater width they can swim around a bit more.
A wider (front to back) tank also allows for better aquascaping. Increased depth that draws the viewer into the picture is more easily done.
There's also consideration for hardscaping (AKA rocks, wood, etc) because you can form a larger base for a structure which extends up the height of the tank, allowing for greater height than would be achieved with a narrow tank (unless you're a risk taker about falling rockwork breaking the tank).