You've got to get into aesthetic dissection to answer this question and I can only give an educated guess. What the japanese are really good at is perfecting shit that other people have invented. Like, they took western gun designs and camera lenses and just perfected the hell out of manufacturing & designing them. Anime began life cribbing styles from walt disney, right? We can assume that, someone please correct me if I'm wrong. So they take this foreign style and put it into the highly efficient cultural evolution machine that is their society.
They took form that style the things that worked best, had the widest appeal. That style had itself been incubated and honed in America for decades, and then the Japanese get a hold of it and by virtue of loss in translation, they take on the style in a very 'pure' way - they really pick it apart and start hammering out what works and doesn't in their own animation industry. They don't have a walt disney to compete with, but they do have their post war boom.
Then in the 60's and seventies, it turns out that localizing these japanese cartoons is profitable! Eventually, foreign audiences start influencing native japanese productions with their dollars. Fast forward to total global saturation.
I don't know if that makes it any clearer - that's my non professional take on the matter.