There are a couple of reasons:
He isn't feast or famine
A fed Varus is weaker than a fed Twitch/Trist, but a struggling Varus does more than them.
Thus he is healthier to both play with and against.
He 'gives back' to his team
Imagine you play Darius top and are looking to TP bot to help your team:
-Varus will provide: CC, Grievous Wounds, decent damage and will provide those throughout the game.
-Twitch will provide decent damage and can carry you in late game.
Varus takes his fair share of the 'carrying' and lets others enjoy their shares as well.
Twitch wants 4 supports.
His damage isn't free
He doesn't burst as hard and has to use spells to really do decent damage, and those are dodgeable.
So he is considered fair, as he gives people a chance to outplay him.
He's has a decent skill cap
While not Kalista or Draaaven, he is still a lot harder than low skill floor marksman like Twitch, Trist or Kog.
This makes him more bearable to 90% of the playerbase.
It also adds 'At least my opponent is good' clause, which most hyper-carries lack.