question

To speculate means to think about something. It also has a special meaning which is to take a chance with an investment, "to bet," as it were, that one will make money with an investment--but that does not apply here.

Speculate in the sense in which it is used here means to think of why something may be so, and it usually implies an informed thinking--if i speculate on why soccer is not as popular in the United States as it is in other countries, i will think about it while considering other aspects of the popularity of sports in the United States.