Because you need to find a new artistic direction.
He was annoying to deal with prior to that ridiculous change, now he can kill an ADC in two seconds instead of four. You gave a tank, one that has burst and sustained damage, more damage. If Nasus's kit wasn't already nonsense prior to this change, I wouldn't be annoyed with it.
Nasus has innate life steal. I mean, this is a rediculous passive for an infinite scaling champ that hits like a truck and can tank like one as well.
Then his Q. Bonus range, 20 mana, and it can take 60% of an ADC/APC's HP in one hit. Forget poor Sona, Nasus one shots her. Not to mention this is an infinitely scaling ability, meaning it gets ridiculously powerful late game. It's also an AA resetter. How fun.
And then we have his W, the bane of his issues. Why the hell does he have an AS slow? Nasus is almost impossible for 99% of the ADC roster to fight, add in that slow and almost no ADC can touch him if he has flash and his ult. This is a 5 second MS and AS slow that has an 11s cd, no cdr, which most Nasus's buy. That's a 6CD, essentially. Hello?
His E shreds armour, as well. Joyous.
On top of all this, he can Q turrets. WHY CAN HE DO THIS WHEN YOU STOPPED RENGAR FROM DOING IT AGES AGO AND RENGAR'S DID LESS DAMAGE AND AHD A LONGER COOLDOWN THAN NASUS'S Q??!?!?!?!?!?!?!?!? Now, with his new ult, he can take down a base in less than 6s at level 18 with 450 stacks.
He had an anti-fun kit, you did not have to change him. He was annoying, but he wasn't like he is now.
What you did is like making Veigar gain all the mana back and then some when killing a minion with his Q.