The reason the term “soccer” is used in countries like New Zealand, Australia and the United States results simply from the need to differentiate that form of football from other forms that either were, or are, more popular in those countries.
Before soccer became widely played New Zealand, for instance, rugby had usurped the name “football”. In Australia the situation was similar, with both rugby and Australian rules laying claim to the name. In the United States it was gridiron.
To avoid confusion, a separate term had to be used for association football in these countries. What more obvious name to use, then, than the already known and accepted “soccer”?