This was because Emperor Ziwei had a good relationship with Ye Xuan; had it been someone else on the throne of the Heavenly Emperor, Emperor Ziwei would have long since drifted away. This also indicated that the Heavenly Court was excessively weak.
Expanding the power of the Heavenly Court and forging a supreme Heavenly Court was the first thing Ye Xuan aimed to do after ascending as the Heavenly Emperor.
Ye Xuan was very fond of the saying, "No school is without teaching, and all Immortals come to court," because this would allow him to rapidly increase the strength of the Heavenly Court.
Of course, Ye Xuan wouldn't just recruit anyone. If it were like the Jie Sect of old, though extremely powerful, its mixture of good and bad disciples eventually led it to decline.