In part as you said it's about "artificial scarcity", but also about rewarding players who have been playing.
I'm sure they have run the numbers and figure this is the best method for them to make the money they want and leave a value for players who have been playing.
Many of the new characters are more specialized and more powerful, so they may want them to be behind a bit of a grind wall, rather than have someone be able to buy them and unlock them right away.
By not offering packs after they also add to the draw to buy as it is the only quick jump you can get on a toon.