Because it is very short and clingy - you have to have a superb figure, great arms and legs, with no rolls of fat anywhere, to carry it off. Most 40 year old ladies wouldn't be able to do so - lucky you if you can
Bodycon dresses are very flattering, they are designed to hide lumps and bumps (for the wearers that have them).
As Tasty Delight said, she's 5ft 1, so 8 inches shorter than that model wearing it so it will not be as short.
Edit to add, a bodycon wouldn't look good nor fit properly if the underwear fits badly. With properly fitting underwear they can make many different shapes/sizes look good. Especially the optical illusion ones, as they create the illusion of small waist/curves in right places.
Underwear is very important in making a dress look good, its amazing how often its overlooked.