Honestly, I think a big factor in why he left is because his shows recently haven't been getting the views Nickelodeon wanted. Fairly Odd Parents ran out of steam as its viewership has decreased significantly in recent years, which is understandable for a show running for so long. The low views started to affect FOP's budget in the animation (Flash animation) and I suspect even the writing (ex: Cosmo and Wanda started to always lose their wands to lengthen the plot). Bunsen also clearly didn't get the views Nick wanted since they shipped it to Nicktoons in less than a year (Seriously, I think Bunsen might have been shipped faster than any Nicktoon I've ever seen! But now it looks like Welcome to the Wayne will get shipped even faster!) Even TUFF Puppy didn't get the most views and Nick basically gave up on it after like 2 years. This is sad, but it's good that Butch said he was very thankful for his opportunity to work at Nick for 20 years. I wish him luck in his future projects.