Whetever the reason, the entire crew was out of contact with the Federation for several years due to the time dilation effect. It was only the fortuitous events over LaPig which caused Spock to make up a new formula for cold fusion and return Enterprise to the correct place in the time-line. (TOS: "Time Travel Invented in Last 30 Seconds of Ep; Never Mentioned Again".) It was especially fortunate for Kirk, as he had engaged in a number of liasons over that period of time and thus avoided any number of paternity suits due to the fact that the children were never born.