The same reason why Vampire is a darkness card. The theme.
Witches use to deal with magic. Spelldamage is the logical consequence.
What theme?
One is not to confuse two categories. One is the theme, one is the mechanic.
Vampire fits Darkness because the Vampire ability fits Darkness perfectly. This has no ability that has to be associated with Darkness.
Witch > Spelldamage makes sense. Spelldamage > Darkness doesn't.
For the same reason, one wouldn't want card drawing abilities in Life, unless they were somehow linked to healing or the number/strength of creatures.
Your critics can be used on a lot of cards...
Following your way of arguing, I wonder why Darkness has got a Quanta Control creature (I know the description of Darkness). Or why the Lycanthrope is among Entropy cards. Or why the Mind Flayer is a Water creature. No offense, but your words remind me of a fusspot
To be more accurate:
No ability associated with Darkness? RT-effect is not associated with any Element imo. Not the Skeleton nor the Mummy would fit. One might say.
-> RT reverses death back to life. RT reverses a nymph that broke bad back to her origin.
Spelldamage and Darkness don't? Steam Machine does neither fit to Water because Water and Fire don't fit at all. One might say.
-> Steam is vaporised water, by heat (fire). I see a connection. As you claimed, witches can do spell damage. I see a connection. As I mentioned above, the Old Hag is a creature that turned from the right path to the dark side. That's obviously the idea behind it, at least it seems to me that this was the thought behind it.