Why Japanese submarines struggled to reach Allied trade

Why did Japan not use its submarines more effectively against Allied merchant shipping?

Japanese submarines did attack commerce, especially in the Indian Ocean, but too few boats had to cover enormous distances while Allied routes could avoid the areas Japan could reach.

Japanese, German, and Italian submarines did attack merchant traffic in the Indian Ocean. The difficulty was concentration and geography: Japan did not have enough boats to cover every task, while Allied traffic could disperse across vast routes, run far south, or operate beyond practical Japanese reach. Japan also avoided attacking American shipping bound for Vladivostok because provoking the Soviet Union would worsen its position around Manchuria. Commerce warfare therefore required access to predictable shipping lanes and a willingness to accept the diplomatic consequences of attacking them.