Well, for a couple of reasons. For one, parts of the story don't make sense. For example, the game is about saving Princess What's-Her-Face but what's that got to do with ruining a gold fish's day at the end of that one underwater level? Why do I have to beat the snot out of...the snot three times? Why should I have to help Pete, for Pete's sake?
The second is the gameplay. The level design is all over the place and can be a maze at times especially in Intestinal Distress. It introduces new mechanics and never brings them back in any level afterwards like the timed water segments, the snot fights, teleporting/mobile crystals, etc. One I especially dislike is the previously mentioned water segment in which you are in a fragile glass container and have to navigate through the water under a certain time. I found this very counteractive as there are also secrets you could collect but if you do so, you will most likely run out of time. If you try to go fast in those segments, you will most likely hit too many walls because the turning is slippery.
Finally the soundtrack. This song pretty much sums up my thoughts on it.
https://www.youtube.com/watch?v=BixpgvPwdHA
I get that this game is going for comedy for everything but I personally don't find it to my liking.