Why is rapid firecannon so good on volibear?

Rapid-fire cannon can be used to increase attack range which also works on melee champions.

One reason it is so good is because of the way the glacial origin and Volibears ultimate 'Thunder Claws' works. If you look up on https://tftactics.gg/champions/volibear you will see that his ultimate "Empowers his attacks to chain between enemies, applying on-hit effects."

The applying on-hit effects part is important here as it means that the chain from Thunder Claws can also apply the glacial origin and potentially if you are lucky enough stun everyone within the chain every single auto attack.

By using rapid-fire cannon you are able to position Volibear to not be on the front line and take aggro, giving yourself more time to ramp up the mana to pop Thunder Claws use it as long as possible while your front line tanks all the damage, and if movement of the unit is required you are able to start attacking quicker due to the range.

Edit: As @XtremeBaumer has said, this seems to be a bug, and for some champions it is, Volibear used to be able to attack across the map due to it using the range from Thunder Claws as the attack range (Still not fixed for Nidalee as she gets human form range in cat form), this was fixed for Volibear before live release and now he is able to attack from 2 hexes away as melee range is 1 hex. This may be nerfed in the future and not usable on melee champions for example, but we will have to see.