>>9680573TMNT has a lot of goodwill built up. It was very popular for numerous generations now, and has had an almost nonstop stream of toys (and now, excellent third party content). The concept is fun and lighthearted, and lends itself well to goofy concepts like movie monster Turtles, or Samurai turtles, or gritty noir Turtles. Everyone has their favorite Turtle, and team completionism means you'll probably make at least four figure sales on any given line.
Godzilla is a much harder sell. The fans seem to mostly be an older set, the toys are almost all super expensive, and a lot of the monsters just don't have a lot of personality by nature. I don't know many young people who are into Godzilla (whereas I know many who like or are at least familiar with TMNT), so the franchise doesn't seem to be enduring as well. There's also not as much fun to be had on the toy side. Sure, you'll occasionally get stuff like the MechaG x Eva stuff, but it's nothing compared to the amount of fun and funky collabs TMNT has done.
