Why did America want an open door policy in china?

Answer 1

The U.S. wanted access to Chinese markets for its products. Ironically, the tables have turned now, and the U.S. is the open door for China, with far more imports coming from China to the U.S. than vice versa.

Answer 2

The US wanted to keep the ability to trade with China free and open - a good amount of imports were coming from China at the time, and one of the key points of the developing country was economic reform.