What do you think 'killed' Rugrats?
I mean it went from being the most popular show on Nickelodeon in 1998 to being off the air a few years thereafter. SpongeBob SquarePants has been on Nickelodeon since 1999 and it is showing no signs of going out of style anytime soon.
In my opinion, Rugrats just wasn't the same after it was brought back in 1997. It seemed more like a 'kiddie show' rather than having some of the adult humor the 1991-1994 episodes had. It was like a different show that had lost its charm. When they added Dil I think that just made it 10x worse. I liked Kimi but by the time she came along, Rugrats had lost it's magic.