In WoW instances, why does everyone think imps are better than voidwalkers for destro warlocks?

Damage. As a Warlock, Your role in instances is to damage and kill as much as possible. While soloing the Voidwalker might often prove most useful, but the Imp deals the most damage of other minions through his firebolt. You don't have to worry about aggro, if you are attacked then the tank isn't doing his job properly, holding aggro is his concern! So if the tank is holding aggro well enough as he should be, the imp will be the best for you as he deals the best damage, improving the contribution of your role.