I believe the answer is: They worried about having to take part in future wars.
At that time, the united states senates prefer to take the position as a neutral force that want to avoid any intervention toward other countries' problems. This stand is taken away the moment the Japanese army decided to initiate an attack to the pearl harbour.
Many of these people began to look for ways to invest their new-found wealth. One of these ways was to invest in colonies. England also looked at the settlement of colonies as a way of fulfilling its desire to sell more goods and resources to other countries than it bought.
