Jack Capellini,
Part of it was the times in that the country had never seen anything like those films before. But what stood out was subject matter pertaining to current events and broad themes. Everyone knows what the original stood for, but then you have Rodan which was a take on PTSD and aerial effects. The Mysterians was an alien invasion movie and one of the first of the genre with a mecha--Less monster movie more science fiction. King Kong vs. Godzilla was a self-aware satire on Japanese television and adopted capitalism--Something that carried over into Mothra which was the real game changer. Suddenly the films were leaning more into fantasy and invited Godzilla into that realm with Ghidorah: The Three Headed Monster, by which time the films had a particular Cold War slant.
Keep in mind a lot of these films weren't directly concerned with monsters fighting monsters, but how everyday people with various jobs (coast guards, scientists, fishermen, coal miners, marketing agents, reporters, detectives, etc) reacted to the appearance of these creatures. This is particularly true for solo monster movies.
Also consider these films weren't trying to build a world, but focus on being the best films possible unto themselves. This allowed for a wider array of tones, atmospheres and topics across multiple movies unconcerned with continuity. When you have the intention of creating a "shared universe" from the get-go you immediately pigeonhole yourself into a tone that every subsequent film must adhere to barring minor variation.
"'Nostalgic' does not equal 'good,' and 'standards' does not equal 'elitism.'" "Being offended is inevitable. Living offended is your choice."
