Because every dwarf needs a big weapon to make up for his small stature.
About this creation
The dwarven ballista was the first 'crossbow' invented. Originally built as a more direct-fire counterpart to the catapult, it soon became the monster-slaying weapon of choice for anyone with cash and a big enough cart.
This belongs to my generic as yet unnamed fantasy world. But it doesn't have elves! Lego has now released an elf hair piece - so now it does.
So, the dwarves invented the ballista, the gnomes stole the design and made the crossbow, the dwarves stole the crossbow design and made the heavy crossbow, and the gnomes then refined their crossbows to make the repeating crossbow. Oh, the joys of inter-racial invention.
The ballista is the best* anti-dragon weapon currently availiable.
*The dwarves are not responsible for any injuries and/or death sustained while attempting to fight a dragon with a ballista.