Doing some research, it seems that the contract was cut and the Simpsons were slowly phased out. Pretty simple but it didn't stop the Simpsons from making a jab at them in "Sweets and Sour Marge"

Still, Butterfingers to Bart would have been like carrots to Bugs.

I know, I think Butterfinger just underestimated how big The Simpsons were tbh.

I’ve seen the commercials for them on YouTube and I’m wondering why I haven’t seen a new one

Butterfinger could have been to Bart what Carrots are to Bugs.