I personally think it's because of how Nasus as a champion himself works.
In my experience, the only way to shut down a Nasus is to dumb everything you have on him early on, especially if it's a matchup he doesn't really struggle against. But given the fact that I'm a scrub in a scrub division, it usually ends up to a cheesy level 2 gank from which if it ends with me either getting killed or using flash. Both of those cases are positive for a Nasus, since me playing with the constant threat of getting camped(you can ward everything but you'll have to fall back eventually) and his unbearable slow(don't forget his shredding too) eventually set me farther behind.
A good Nasus will sit tight, freeze lane under his turret and stack untill hell freezes over. Then he'll dunk you so hard you'll discover your life's true purpose. You could argue that one had ample time to stop him from becoming a monster, but it can be farely hard to deny him of cs he will get NO MATTER WHAT-because even if you devastate him in lane he'll still just stack himself up- because if you aren't carefull or sometimes the champion you play or you are just bad like me you'll end up autopushing the lane in an attempt to deprive him of cs. In addition to the constant threat of ganks(again, you can ward, but that doesn't mean you'll join the enemy jungler in a dance party insite the warded bush) it will inevidably lead to a point when Nasus still gets stacks and 1 or 2 respectable items.
From that point on he can just fend for himself due to his kit and itemization even if he is in a shitty situation. Who doesn't love a crippled adc? Who doesn't love AoE shredding? Who doesn't love an On-demand tank that can still do massive damages if left unchecked?
The thing is, he doesn't really lose anything from being shit-on early game(sorry for bming), because his stacks always stay the same. He will just lag behind, but if his team is good enough they ALWAYS buy him enough time to get back in the game, and then you just run for your lives.
What I'd like to see is him losing stacks every time he dies, similarly to Cho Gath's ultimate, but in less severity. Imagine if Cho just got max stacks for the entire game. If you had a hard time dealing with him when full and then setting him back by eventually killing him, he came back again with full stacks only to go through the same proccess again. Let me iterate: just like the way Nasus's passive scales with levels(I don't remember the numbers) what if his Q lost some stacks as well? In the beggining it could be 5 stakcs, then 8-10, then 15-20, that way he doesn't get to 700 with a trinity despite having died like 8 times already, just because his team was smart enough to buy him enough time?
sorry for the spelling mistakes I didn't bother to proof-read