Perhaps in the original timeline, Fritz was the original time traveller chosen by the Entity to defeat Lavos. But then when he got to the Sewers in 2300 AD, he was all "Man this is way too complicated for me to figure out how to press two buttons, evade these sewer monsters, and get the hell outta here. But you know, I kind of like it here and I never really wanted to defeat that porcupine thing anyways".
The Entity, surprised by Fritz' arrogant abandonment of his epic task, decides to try for a Plan B with a spiky haired kid that plays with swords too much. Luckily, things worked out better the second time around.
[/end sarcasm]