According to the Article, what is one reason why President Abraham Lincoln turned down the offer from King Mongkut of Siam to send elephants to the United States?

that the American climate might not be comfortable for elephants

Explanation:

The one reason why President Abraham Lincoln declined the offer from King Mongkut of Siam to send elephants to the United States was due to the fact that the American climate might not be suitable for the inhabitation of the elephants and that American could make use of alternatives which is the use of steam engines as a means of transportation and also as beast of burden.

King Mongkut of Siam who was also known as king Rama iv, was the fourth monarch of Siam, Ruled between the year of 1851 to 1868.