Conversation so far:
When did Pancho Gonzales play open tennis
Gonzales was therefore ineligible to compete at the Grand Slam events between the end of 1949 (when he turned pro) and the start of the open era in April 1968.
What happened in 1968 in tennis for players like Pancho Gonzales?
start of the open era of tennis

Question: why was he ineligible to compete
Self-contained rewrite: why was Pancho Gonzales ineligible to compete in grand slam events prior to 1968?