The problem is that Nasus doesn't need it and it adds nothing to his gameplay. Oh, those few instances of time where you're dueling your top lane opponent and they just TP away. snap Darn, wish I'd played someone other than Nasus. Oh well, guess I get to farm freely now while they're out of lane.
See what I'm talking about? There wouldn't be any point in adding this mechanic to Nasus because it adds nothing to his kit. Why would they implement something like that? I think the reason Nasus doesn't have something like, say, a stun or another form of hard CC (which would include this no-range knockup on his cult) is because he has A) An infinitely stacking damage mechanic, B) A slow that basically turns INTO a stun once he maxes it, C) A big-damage AOE that gives him actually some really nice poke AND reduces the armor of people who stand on it, and D) an ult that boosts his attack range, armor, magic resist, and deals damage to everybody.
His kit isn't meant to be CC overloaded or have very many forms of CC in it period, regardless of what kind or how close he needs to be to someone to actually use it.
You're agreeing that this wouldn't add much at all, so why do you want it so badly? I can understand wanting to play Nasus, but people need counterplea for him considering how hard he scales if you stack correctly. Being able to Teleport away in the middle of fighting him is one of those counterplay mechanics.