A suit is a matching set of a jacket and trouser. It's not enough to say that they match, but must be cut from the same bolt of cloth when the pattern is cut. This is when the suit is just a piece of flat fabric. This part is essential in the manufacturing process to ensure matching die lots. If you cut the pant from another bolt of fabric it may not match even though it is the same.
What also must be realized about a suit is the sizing. You cannot buy a 40R jacket with a 38 inch pant. Usually and depending on the drop of the suit, a 40R US or 50R EU comes with a 34 inch pant. In Europe the jacket and pant are the same size. A 50 suit jacket comes with a 50 pant. When purchasing a sport jacket and pants you can choose different fabrics, colors, patterns and sizes. They all should coordinate together with any other accessories to create an overall image or "look."
Suit jacket styles are also more limited in terms of design than sport jacket styles. Sport jackets may have patch pockets, bi swing backs and leather ornamentation. All things you probably will not find on a suit jacket.
This also hold true for fabrics. Certain fabrics you may be able to use for a sport jacket when you cannot use it for suit. The reason being that some fabrics are too delicate for the wear and tear the trouser will encounter during its lifetime. For instance, silk and wool combinations are notoriously good for sport jackets, but not for pants. The silk is too delicate and the pant could wear out in high traffic areas of friction. This could occur on pocket edges, in between the thighs and on the waistband. Some sport jackets are made with heavier fabrics in winter. These may be too heavy for pants.
In conclusion, there are fabrics and patterns specifically made for suits and others specifically made for jackets. I have made suits out of jacket fabrics, but the composition of the fabric is critical at that point to be sure the pants will last the test of time.