Why were the battles fought in the Pacific theater of WWII more difficult to fight than those in Europe?

The battles in the Pacific theater of World War II were more difficult to fight than those in Europe because they were fought on islands.  This made it very difficult for any attacking forces.  In Europe, battles were fought on a large land mass, allowing attackers to maneuver and avoid the sorts of battles that the Allies faced in the Pacific.

In the Pacific, the Allies had to fight their way through a large number of small islands.  They fought on tiny islands like Peleliu, Saipan, Iwo Jima, and Okinawa.  On those islands, it was impossible to maneuver around Japanese forces.  They could dig into prepared defensive positions and the Allies had to come right at them. The Americans had to fight the Japanese cave-by-cave through some areas of these islands. This was terribly difficult.

By contrast, the war in Europe was more of a war of maneuver. The invading Allies typically had more choices of where to attack.  They did not have to mount head-on attacks against well-prepared defensive positions. Because of this, the battles in the Pacific were much more difficult than those in Europe.