Explain Why was fighting the Japanese so difficult for the allied soldiers?

Because Japanese culture is so radically different from western culture. The emphasis on individual sacrifice for the larger group is at the core of everything. Soldiers were willing to do anything at all for the greater good....in this case winning the war against the US. The level of personal sacrifice was astounding and extreme.
The classic example of this was the Kamikazi would felt that it was a great honor to be chosen for suicide missions, apparently no one hesitated to do this. There are many other examples of this over zealous enthusiasm......it's really an over the top attitude and the allied forces were at a loss. It didn't make any sense.

Because the Japanese simply refused to surrender.They believed that the Emperor Hirohito was a god, he was divine. They truly believed this, they simply felt that Japan was invincible and no matter how bad things got they were absolutely sure to win the war. They had no doubts long after it was clear to the world that they had no chance whatsoever of victory.
Surrender was so dishonorable and unthinkable for them that when they finally did surrender to the allied forces....many, many Japanese committed suicide, and not just soldiers. The shame was too unbearable for them to live with.

Some believe that if Hiroshima and Nagasaki weren't bombed with the first nuclear bombs ever used....that the entire Japanese population would have fought to the last man women and child. Some believe that dropping these bombs actually saved millions of Japanese lives. We'll never know, maybe that's true. The fire bombing of Tokyo and other cities by allied forces was endless and is one of the most brutal aggressions against civilians in human history....and it didn't put a dent in the resolve of the Japanese. They WOULD NOT GIVE UP!

Also, the Japanese had the Zero-1 fighter planes.....which were ahead of their time....and they had LOTS OF THEM!

There are probably lots of other reasons but that's all I can think of right now.

Japanese people are different from the rest of the world in my opinion, very difficult to understand how their minds work.

Personally, I was married to one for 14 years and I'm not impressed.

Hope that helps.