Why is the humpback whale on the endangered species list?

The most common reasons for the Humpback Whale's listing on the US Fish and Wildlife Species Endangered List is due to over commerical hunting/fishing, entanglement in fishing nets, collision with ships, distrubance of migration routes by shipping vessels, radar and sonic testing in migration and protected areas, decline in food supply from over fishing or global changes. There are several other factors, such as solid waste dumped at sea, ocean temp. changes, etc, but many researchers can not agree on their actual influence. For more information and details see sites listed below
People used to go whaling or whale hunting, but now that is against the law in most countries.
Because of Hunting. Man has severely affected humpback whale populations by killing them for years. By the 19th century, many nations (USA in particular), were hunting the animal heavily in the Atlantic Ocean, and to a lesser extent in the Indian and Pacific Oceans. This widespread hunting sharply reduced whale populations.

It is estimated that during the 20th century, at least 200,000 humpbacks were taken, reducing the global population by over 90%, with North Atlantic populations estimated to have dropped to as low as 700 individuals. To prevent extinction, the International Whaling Commission banned commercial humpback whaling in 1966. By that time the population had been reduced to around 5,000. That ban is still in force