Japanese monster movies don't look like other monster movies. There's a whole different aesthetic going on. They truly look surreal and the filmmakers don't care- they want to tell a good entertaining story, realism be damned. That's what grabbed many of us older fans as kids. It's that unique style and look that the suits had. Sometimes they looked like giant animals we knew- a spider, a butterfly, even what we thought dinosaurs looked liked then- but sometimes they would just be completely different.
Especially in the later 60s and on into the 70s. There were television programs in Japan showcasing superheroes fighting giant monsters and the designs got further and further out there. Megalon or Gigan would never have appeared in the early 60s movies. There's only so much you could do with regular giant creatures, I guess.
I love those wilder designs. The movie monsters never looked too silly, whereas some of the TV shows could and would get a bit laughable. In the context of the show they worked, especially since they were treated with respect from the actors.
I think that's been missing from the the movies these days. The giant monsters in PACIFIC RIM are not really kaiju, despite being called that. Kaiju are unique.