Because the rules of nomenclature say so.
First, you need to name the first element. You can look it up on the Periodic Table of Elements. Then name the second element by using its root + suffix. (Note: -ide is most often used.) For example:...
1- Mono 2- Di 3-Tri 4- Tetra 5- Penta 6- Hexa 7- Hepta 8- Octa 9- Nona 10-Deca
1: mono (but only used for the second element) 2: di 3: tri 4: tetra 5: penta 6: hexa 7:hepta 8: octa 9: nona 10: deca
Almost always. Especially when there are more than one atom of the first element in the compound.