The Japanese were very well defended on Iwo Jima. They had prepared an extensive system of underground bunkers and pill boxes.

Japanese commander Kuribayashi did not think that he could win, or prevent the Americans from taking Iwo Jima. He was determined instead to kill so many Americans that it would make them think twice about attempting to invade the home islands of Japan.

Taking lessons from the Battle of Peleliu, Kuribayashi decided not to fight the Americans as they came ashore on the beaches, but rather to make the interior of the island into a giant death trap of pill boxes, bunkers, land mines, and hidden artillery and mortar positions. The defenses were laid out so that every part of the interior of Iwo Jima was exposed to Japanese arcs of fire.

The defense is generally held to have the advantage over the offense in warfare. How much of an advantage has varied in different conflicts. In WWII it was a rough rule of thumb that the offense needed at least a 3 to 1 advantage in numbers (and preferably much more) to win.

The US threw about 70,000 troops into the battle to take the island. The Japanese had 22,000 troops defending it. The Americans had a 3 to1 advantage, but only just.

The battle for the Island took 5 weeks. The Americans suffered 6,821 men killed and 19,217 wounded. The Japanese had 18,844 killed (either by combat or suicide) and 216 taken prisoner. There were also about 3,000 Japanese still hiding in deep bunkers when the battle was officially declared over. These either committed suicide, were killed, or surrendered after the battle. (Only 1083 Japanese troops survived the Battle of Iwo Jima.)

http://en.wikipedia.org/wiki/Battle_of_Iwo_Jima

The Americans had more total casualties in the battle than the Japanese (but not really "so many more"). Many of the American casualties were wounded rather than killed. Almost all of the Japanese casualties were killed.

The Battle of Iwo Jima was very hard fought and bloody on both sides.

Source:

http://www.history.navy.mil/library/online/battleiwojima.htm

Iwo Jima was part of the Bonin Island chain. This island chain needed to be secured before any invasion of mainland Japan could be considered. The Americans knew this and more importantly so did the Japanese. They evacuated the small civilian population and began to work on extensive fortifications, and built an extensive underground defense network on the island. The Japanese commander, Tadamichi Kuribayashi wanted to let the Americans land unopposed and than inflict serious casualties on them,while kamakaze fighters took their toll on the landing fleet. The Americans knew before hand how serious the defensive fortifications were (a proposal was even made to the president to use poison gas on the island) but since the island was strategically necessary they went ahead with the landing. They tried to bomb the island first from the sea and air but it had little effect. The Japanese were also determined and refused to surrender because they knew the loss of the island would have major ramifications for the home islands.To give an idea how determined they were: only 216 out of the 20,000 Japanese on the island were captured alive.

So, you are correct, the reason the casualties were so numerous was because of the numerous fortifications and the determined Japanese resistance.

A good book to read on the pacific war and the war in general is "A World At Arms" by Gerhard Weinberg.