ATP and WTA are private companies and essentially the core of this sport. ITF organizes the Grand Slams and Davis/BJK Cup but those are secondary events in largely individual sport. This is like those major leagues in USA, like FIBA could never intervene into NBA which has even adopted slightly different rules to international basketball over the years. There is no global government body that technically means less than ITF in tennis. If they alienated the top players nobody would come to watch some ITF 50k stalwarts playing in Masters-level tournaments and the money loss would be huge. Wimbledon thought they are bigger than tennis but nobody followed them and I believe All-England club is already considering the change on the ban given serious financial loss - not to mention from a PR stand point.