Chinese and Japanese are enormously different spoken languages. Except for a large number of words imported directly into Japanese (but evolved to sound quite unlike the originals), the two languages have essentially nothing in common. The pronunciation, the grammar, everything is 180° different. A consequence of this is that those Chinese characters, evolved over millennia to fit the Chinese language like a glove, were a bad match for the way the islanders spoke.
Imagine those poor scholars of the Yamato court in Western Japan in the 7th century. They must have been intrigued by this revolutionary technology called “writing”, where you could freeze your words onto a stone or the blade of a sword so that others may understand it later. Why leave it to the Chinese immigrants? Why not master it for their own native language?
Except it must have been excruciatingly difficult. The characters were meant to be used as modular building blocks—a kind of modularity that Japanese just didn’t have.