Because they are 880 mana to make, require level 45, and cost 6.5k a backpack at the shop which is alot :O
The mana for the runes costs like 3x 7.4? (not sure exactly how much..)
It's 4x the original mana requirement, but the regen is also boosted 4x, so it takes just as long as 7.4 to make runes.