Problem is, Nasus can afford to take quite a few hits while farming, and since his Q only hits one creep, he can freefarm pretty easily.
Veigar does not have that convenience. First of all, he's not tanky at all. If he burns his Q for poke in the early game, he has to wait 7 seconds to get it back. Secondly, if he is farming with Q, he needs to get 2 minions in a row to less than 10% health, then Q both to safely get 2 stacks, all while trying not to get poked down by Leblanc or whoever. This also keeps Veigar from being able to freefarm as he pushes the wave WAAAAAAY more than before. Nasus barely pushes the wave at all and his stacks don't require a skillshot.