Main reason is feeling. It feels cool if it is more difficult to tap into Apsus dreams than it is to observe some flame or the flight of birds.
Asking spirits is easy, if you can compel them to speak. Dark knowledge is therefore more expensive, but easier to research.
In short there is no real reason from a game point of view. Or, the reason is the same as why spells aren't identical in dmg and dmg type. It would be boring if all spells of lvl 3 did an equal amount of fire dmg regardless of path used, or if all summoned critters had the same stats on the same lvl and only differs by sprite.
In short, it gives differentiation to the game. Also I find it cool to make things assymetrical
