They are poached for body parts that are used in traditional Chinese medicine. Tiger balm is popular one of these medicines. People in China believe that anything named for a tiger has healing powers because it is likely to have a part of the tiger in it. Tiger parts are used in this medicine because it is believed that they have healing powers. There may be no scientific proof that tiger parts have healing powers.
Habitat loss is another reason for the decline of the tigers.
The forests of Asia have been used for agriculture and lumber. Without, forests tigers have nowhere to live, and nothing to eat. Another reason for the tigers decease is the population fragmentation. The loss of the large forests prevents tigers from roaming long distances. Because of this they must stay on their small forest island.