Very long story short, farming and sugar, as well as a port to china, east india and asia. and the islands were a prime place for a navel base.
Immigration was certainly a key factor in the growth of America's economy . The Industrial Revolution came with massive technological developments, and the chance to exponentially improve the production rate of factories. However, to operate these emerging industries it would take a tremendous amount of laborers. American citizens working alone, weren't going to be enough. In other words, the peak of immigration influx allowed the U.S. to take full advantage of the possibility at the right time.
Hope this helps!
The major reason for this was the fact that many Senators were afraid that membership in the League of Nations would reduce the sovereignty of the United States and its ability to have complete control over its own foreign policy and military actions.
