ok, ease of implementation for one, and two, to allow new strategies. These cards are other and therefore playable in any deck.
So, you can have a creature with momentum in your Aether mono, or have a creature with Devour in a pure death deck. it might be more complex or expensive than adding pendulums or it might be a bit more streamlined, the idea is to get a few abilities, mostly ones that have been around awhile not brand new ones like overdrive or obsession, but dive and scavenge and such spread around a bit to spur some ideas and tactics.