As mentioned, it's all about gameplay reasons. A crafting skill in each of the three archetypes. (Also, poisons.)
Also it's not like Speechcraft and Mercantile being stealth skills makes sense. (No idea if they changed that in Skyrim though, I don't really keep myself up-to-date on this.)
By the same token, why is Speech a "stealth" skill? This seems awfully forced.
Streetwise, coercion, bluffing, lying, negotiation, subterfuge. Politics. Diplomacy. ("Diplomacy is the art of saying 'nice doggy', until you can find a rock." :tongue:)
Of the three basic archetypes, Magic Combat and Stealth, Speechcraft fits into Stealth the best by
far.